There's few conditions that have to be meet for a successful interruption:

1st. Was opponent casting from position in team that was making it possible for your interrupter to success? aka- never try to interrupt position 1 ninja from your position 2 or even 3 (saw some magicians trying to do this).

2nd. Ninja which is trying to cast mystery might have immunity for interruption with that certain skill. Keep that in mind.

3rd. Going back to positions- even if your ninja is on same position- you still need higher initiative and/or good reflexes to succeed.

4th. Connection might still be a thing- either your or your opponent's.

Generally speaking, I face much less problems with all kind of delays after the introduction of 5.0 so maybe it's worth to look a bit deeper on the situation- maybe problem lays somewhere else, not in game itself.

This.

Interruption needs great timing- even if your Iruka is on position 1- and opponent tries to cast from position 3- if you won't do that in right moment, you may fail it. It's like a one second difference sometimes which determinate if you will be able to stop opponent- the second in which the game already proceed opponent skill.

Non-prompt skills are easier to interrupt- especially if opponent don't have a great timing with casting it.

And also this- interruption is nothing like some control statuses- for example immobilization. It don't leave any effect on opponent's ninja if you cast it earlier- so it's usually all about precision and reflex of caster.