No xp gained when running map

I'm running a level 89 pathfinder at the moment, and I'm running tier 15/16 waystones. I don't know why it's happening but on something like 1/5 maps I get zero xp when running the map. I'm not dying, I'm completing the map, I've killed all the monsters, and yet when I leave the map I'm on exactly the same xp as I was when I started. If I can find a pattern as to what's making it happen I'll come back and post more info, but for now I'm just baffled.