If you've noticed that your mattress topper is sinking in the middle, there are a few possible explanations. It could be simply due to the fact that the topper is softer than the rest of the mattress and therefore sinks more easily under body weight. Another possibility is that the topper isn't evenly distributed on the mattress, which can create an uneven sleeping surface and cause sinkage. Finally, it's possible that your mattress itself has dipped in the middle over time, causing the topper to follow suit.
If you're concerned about your sinking topper, start by checking to see if it's evenly distributed on the mattress. If it's not, try adjusting it or flipping it over so that it's more level. If this doesn't help, take a look at your mattress itself to see if there's any dip or sag in the middle. If there is, you may need to replace your mattress entirely. Otherwise, a new mattress Topper may do the trick!