If it's a really short cast bar... well, as others have mentioned, 'lag.' Think of it as like locking in the boss's stats for the ability... if you don't apply it soon enough, it won't get calculated in.

If it's something with a really long cast bar, well, take your time. FWIW, with revelation I usually adlo *then* virus (an as example). Technically I should probably get in the habit of doing it the other way around, but the cast bar is long enough that virus at 25~50% still takes effect without problem.