Hi jhandy,
There could be many reasons for this including thicker layers of wool which will compress (and this is a normal part of thicker wool layers), other materials used in the comfort layers (such as lower density polyfoam), choosing a mattress where the comfort layers were too thick and/or soft for your body type, sagging in the foundation under the mattress, or some combination of these. Of course it’s also possible that there was a manufacturing defect in the foam itself (which tend to show up early in the life of a mattress) and if the visible impressions (with nobody on the mattress) is more than the warranty exclusion then it would be replaced under warranty. It’s difficult to speculate about which of these may have been part of the cause without knowing the specifics of the mattress and circumstances.
You will find reports of almost any mattress sagging for different reasons but it can be difficult to know the specifics of each instance without knowing more about all the factors that may be involved or whether the sags are visible or are “virtual impressions”. You can read more about all the factors that can affect the useful life of a mattress in post #4 here and the posts it links to.
The best way to avoid this is to make sure that you know the specifics of all the materials and components in your mattress so you can identify any “weak links” in the mattress and to choose a mattress that is well inside your range in terms of PPP. If a mattress is “on the edge” of being too soft for you when you first buy it then even small amounts of foam softening or the compression or breakdown of materials in the mattress that would be “normal” for the materials can lead to the premature loss of comfort and support and the need to replace the mattress.
Phoenix