these have all been great advice that you need to take on board
what i have notice is that if i buy a new plec i will add food but i would not expect it to eat it (some do) if they have caves set up hidding places around in the tank perfict water conditions and other younger plecos in there
but all and all pleco's are not very active fish they keep to there selves and when your not around they are doing there thing
you need a test kit
you need to read about the species
provide it the conditions it requires
plus correct food
maybe a partner in the future
and even with all of this it will still not be active it will still be scared of you when you walk up to the tank but maybe one day if you have done all of this and not harrased it it might come out when it see's you if it does tho ever please tell me

how lol
i only see my breeding plecos out and about if there in an comunity they are always hidding
make sure you are doing small daily water changes with vacuming parts of the tank till you get another filter to help out the one you have.
good luck and dont stress out to much keep lights off and try not to bother it .
let us know what is the out come