Hi abbygrant,
I would keep in mind that here will be a break in and adjustment period for any new mattress or sleeping system as the mattress loses any of it’s “false firmness” and the cover stretches and loosens a little and the materials settle and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here). This would typically be a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress (higher density materials can take longer) and it can be surprising to many people how much their sleeping experience can change over the course of the first month or so and I would tend to sleep on a new mattress for a minimum of 30 days or so if possible before deciding on whether to exchange or return it (or possibly add a topper if the mattress is too firm).
Some of the suggestions in post #2 here may also be helpful because the support system under your mattress or any mattress pad or protector you are using and sometimes even the temperature in your bedroom can also have a significant effect on the feel and performance of a memory foam mattress.
Again I would try and sleep on any new mattress for a month or so before deciding what to do.
Unfortunately it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because you are the only one that can feel what you feel on a mattress and there are too many unique unknowns, variables, and complexities involved including body type, sleeping positions, health conditions, and individual sensitivities and preferences that can affect how each person sleeps on a mattress in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and your Personal preferences) or any “symptoms” they experience so a mattress that would be “perfect” for one person may be unsuitable for someone else to sleep on.
Having said that … there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be helpful as well.
These posts are the “tools” that can help with the analysis, detective work, or trial and error that may be necessary to help you learn your body’s language and “translate” what your body is trying to tell you so you can identify the types of changes that have the best chance of reducing or eliminating any “symptoms” you are experiencing (at least to the degree that any symptoms are from your mattress rather than the result of any other circumstances or pre-existing issues you may have that aren’t connected to a mattress).
The thickness of a mattress or any individual layers inside it is really just a side effect of the design and the design goals of a mattress and is also only one of many variables that can affect the feel and performance of a mattress relative to any particular person and by itself isn’t particularly meaningful (see post #2 here). In some cases higher weight ranges will sometimes do better with a mattress that is thicker than lower weight ranges (see post #14 here for more about the effect of thickness) but even this depends more on the specific design and combination of materials in the mattress and on how well your testing or personal experience indicates the mattress “as a whole” matches your specific needs and preferences in terms of PPP than anything else.
There are certainly many other people that have difficulty finding a mattress that is a “good enough” match for them in terms of comfort, firmness, and PPP … especially if they are closer to the “princess and the pea” end of the range than the “I can sleep on anything” end of the range.
When you are purchasing an online mattress you don’t have the option of making side by side “real time” comparisons with other mattresses to find out which of several mattresses you may prefer or that may be a better match for you in terms of PPP and you will generally find that any specific mattress may be the “best” match for a relatively small percentage of the population, a “good” match for a larger percentage, and an “OK” match for a larger percentage yet.
While the only reliable way to know which group you fit in will be based on your actual sleeping experience … for those where the mattress isn’t a “good enough” match (however they may experience or define this) then the return policy of an online choice can help reduce the risk of an online purchase to any costs involved in exchanging or returning it which are often free or minimal and lets you test the mattress in your bedroom instead of a showroom with little risk outside of the time you spend trying the mattress or returning/donating it (should that become necessary) or any costs involved in returning the mattress (if there are any) and starting all over again with choosing another mattress that will hopefully be a better “match” for you.
There is also more about the different ways to choose the most suitable mattress (locally and online) that can help you assess and minimize the risks involved of making a choice that isn’t as suitable for you as you hoped for in post #2 here.
If you have tried a number of online mattresses and none of them have worked out well for you then it may be a good idea to try some local shopping instead so you can make more meaningful comparisons between many different mattresses in “real time” instead of trying mattresses “one at a time” over a much more extended period of time.
If you let me know your city or zip code I’d be happy to let you know about any of the better options or possibilities I’m aware of in your area.
Phoenix