There is a display limit of 30 debuff icons per mob. If you apply dots on a mob that already has, say, 40 debuffs, your dots will apply, but you will not see them. There is a hard limit of 60 debuffs that can be applied to a mob. This means if there are already 60 debuffs on a mob, and you apply dots, they will not work.

GG Diadem SMNs.