First off, I wouldn't recommend sitting on the bladder haha
Secondly, I've somewhat experienced this issue before. In my case, I realized that the inflator hose was not seated properly when I took it out to wash it. So probably if you (or anyone else) had removed it, probably wanna check that it is seated properly. Rotate it around and see if it shift. Mine "popped" into place when I rotated it and I realized that's the problem. Since yours is fairly new, I don't think it's the springs in your valves getting stuck but it might be worth a try.
Worse come to worse, contact whoever you bought it from. If it is from dge, their return policy is excellent and they'll help you out anytime.
Hope this helps!