How often do you do water changes?
Do you test the water parameters of your tank?
Do you vac the gravel/substrate, how often?
The substrate may be dirty in the area he hangs out in, Plecos can be big poopers, gets a bit too mucky, and he might hide in that corner/spot most of the time and have more bacteria build up in that spot causing some fin rot.
Try cleaning/ vac'ing the spots he sleeps/sits in the most.
Are you familiar with the nitrification cycle?
Make sure to have a decent amount of water dechlorinator on hand and plenty of time.
If you do weekly water changes then do a couple back to back water changes you can do vac'ing in those spots - instead of one pwc
Alternately do the vac'ing till the water level is close to but not low enough to turn off the filter. Then fill up and do more vac'ing, repeat, repeat.
Then if it isn't cleaned up enough then the next day do another vac'ing in those spots. It doesn't need to be spotless but focus the vacing just on the spots he hangs out in.
If you start vac'ing every where in the tank you could disrupt too much muck under the gravel and cause a mess in the tank.
IF you do NOT do frequent water changes then you want to proceed in the water changing more slowly.
This is because the difference in the water of the new tap water is so very much more different than the tank water which has been in the tank and would have a very much higher TDS
due to just adding refill water instead of doing water removal.
Fast / large amounts of water changing after no water changes for a long period can cause shock resulting in death if not made gradually.