Hi SleepDeprived,
Unfortunately this is because Simmons (like most major brands) uses lower quality/density foam in the upper layer of most of their mattresses … and worse yet they don’t disclose the quality of their foams.
As TweedSAFD mentioned … density and “comfort” and firmness are not closely related.
Any density foam can be made in a wide range of firmness levels from very soft to very firm and the main difference between them is that higher density foams are more durable and will maintain their original properties longer even though they can feel the same when they are new if they are the same firmness level. For example a 1.0 lb polyfoam may only last months or even less while a 2.5 lb polyfoam can last for many years (well over a decade). Lower densities are also much less costly.
You can’t “feel” the quality/density of a foam in a mattress but you will certainly feel it in a few years (or less) when the lower quality foams soften too quickly and there are “soft spots” in the mattress. Unfortunately foam softening and the loss of comfort and support that goes with it isn’t covered by warranty unless the foam also has impressions that are deeper than the warranty exclusion and this only happens in a small percentage of cases. This is why it’s so important to make sure you know the density/quality of all the materials in your mattress … especially in the upper layers … because a mattress softens and breaks down from the top down.
With polyfoam I would look for 1.8 lb density or higher unless you are in a lower budget or with a two sided mattress where 1.5 used with caution would be suitable. Higher density will be more durable yet.
With memory foam I would look for 4 lbs or higher. Once again higher density will be more durable as well (up to about 6 lbs where the benefits of higher density starts to lessen)
In both cases I would make sure that there is no more than “around an inch or so” of lower quality/density materials in the comfort layers of the mattress (usually in the quilting) because once you reach about 2" or more then this can become the weak link of the mattress and the risk of foam softening affecting your mattress becomes much higher. In your Simmons it’s very likely that there is too much lower quality/density foam in the upper layers which is most likely to be the cause of your problems. The innersprings are probably still fine.
Any latex foam will be more durable than both.
Density in memory foam and polyfoam is the single biggest factor in the durability of a foam but has little to do with “comfort” or what you “feel”. You can read more about the factors involved in durability in post #4 here.
Phoenix