I've experienced the same thing where Rotpriest is not triggering when it should. Either with it being targeted or other of my creatures being targeted. I have had several instances, but in one clear examples there was only my Rotpriest on the field, the opponent played ossification and then no poison counters were triggered. I also had a different game where I had one additional creature, and the opponent played a Nowhere to Run on the other creature and no poison counters were triggered. It seems like any exile affect is not triggering Rotpriest, but this should not be the case as they are still targeting my creatures. Fixing this would be really awesome since I spent some wild cards to make my deck and this not working makes me sad. I can post a picture or a video if it helps as I'm sure I will run into it again :)

I was recently in a match where I absolutely can not figure out a reason why my rotpriest would have just suddenly stopped triggering. I had rotpriest and multiple other infect/deathtouch creatures on the board, however I had gotten stalled out on attacks with 9 poison counters on opponent. I drew a go for the throat and cast it on my own creature which should have been lethal, except it did not trigger the rotpriest and give the final poison counters. Next turn I drew Gaea’s blessing which again should have been lethal just by casting it on my creature, but with what had just happened I correctly assumed that it would not work and had to send in attacks and give a death touch/infect creature trample & the counter to get lethal in.

I have taken screenshots of the entire board, and see nothing that would explain this.

Ossification is a spell, but it doesn't target your creature (it targets the land it's going to enchant). It's ETB trigger is what targets your creature and that is what exiles, so there is no trigger event for your Rotpriest, since the creature is not targeted by a spell. It's very likely the same reason for the damage spell, but without knowing which card it was, it's hard to tell.

My rot priest did not trigger several times through my most recent fight.

I was ossified (ossification) by an opponent on my Fynn the Fangbearer while Rot priest was on the field, my opponent had no cards which would have prevented Rot Priest anywhere through out the field. Yet still he received no poison counter for targeting my Fynn.

This happened a second time with a damage spell that he cast against another of my creatures, both spells he used clearly said "target creature" and Rot Priest clearly states "whenever a creature you control becomes the target of a spell" I lost this match because of these poison counters not being applied several times. Please fix this bug.

Ossification is clearly a spell: "In MTG, a spell is any type of card cast by a player. Spells are usually cast from your hand, but in special cases can be cast from other areas of the battlefield like your library or your graveyard. Land cards are the only type of card that is not considered a spell." < from your own rules. These were all cast from his hand.

Note: it did work when I cast Tomito's Safe Keeping.

since you did not provide pictures I do not know what cards your opponent had on the field but there are some creatures that prevent triggered abilities from triggering. maybe the opponent had one of those and later it was dead