Hi brittany741,
Unfortunately this is fairly typical of thicker memory foam comfort layers which will get softer under the heavier parts of the body resulting in the loss of comfort or support even though they still “come back” when you aren’t on the mattress and don’t have deep enough visible impressions to qualify for warranty coverage. The underlying cause is usually in the memory foam comfort layers. Mattresses will also develop soft spots more quickly with higher weights.
In general when a mattress has developed soft spots in the comfort layers it’s time for a new mattress but there are a few suggestions in post #4 here that may be helpful at least partially or temporarily until you are able to replace the mattress.
Phoenix