Reply to Help! Deformed beat grid!

So sometimes when I play a song the beat grid all of a sudden is deformed. What I mean is that some beats are longer than others in the grid. Also it doesnt matter if I have already locked the beat grid on the song before. It does not happen regularly, so I cannot figure out what the reason for this is. Is there some menu option that I need to click or did I mess something up with my mapping? Please help before I go insane.