Have they changed the AI on Interrupts?

Mesmer spells like power drain interrupt SPELLS & CHANTS, not skills, like Troll Ungent. Leech Signet should however interrupt those skills, however, it has a long recharge, and thus can be used less often, which may or may not explain why you're heroes aren't interrupting.
My heroes personally are pretty much god-eyed, I rarely have problems with them interrupting and using the required skills when needed. Hence my post - I watched my Heroes bars, and they weren't using their signet to interrupt. (I specified their use of Leech Signet btw. I know that Power Drain works only on spells and chants.) So, it isn't that they had simply used their signet already, and were just waiting on a recharge.

They used to use it prior to the holidays, which makes me wonder if something got tweaked/damaged during one of the updates?

Thanks, all, for your responses - but, it doesn't answer the question of why they used to do it before, and have since stopped. There are no known updates which explain this, and as you see others are having good results with their heroes interrupting. So its 1 of 2 things. Check your interrupt heros' AI setting--needs to be guard, not avoid combat. Just check it--I have randomly checked heroes many times and found that their AI was set differently than where I thought I had it. If its not that, then its just the luck of the draw when you happen to be watching them use their skills. My guess is they are getting off their interrupts, just not when you're watching them.

Load them up with a cheap hex that has a long duration and throw this onto their bar: [skill]Signet of Disruption[/skill] Thanks, Nightow - I'll give this a try! (Didn't even think of this because I was using them as an X/Me whatever w/Power Drain for an interrupt and e-man, plus the Leech Signet so they could help me stop enemy skills. This will mean dropping Power Drain, or another skill from their primary, but might be worth it just to experiment and see if they use it to stop skills.)

I have checked their combat settings as well, and they are set for defensive, not avoid, so that isn't it. Like I said at the start, they used to use it correctly, and now they aren't. Frustrating as hell when I am H&Hing areas with lots of Lava Driders (Troll Unguent) or giants (Giant Stomp) and I'm the only one using my Leech Signet! (And, yeah, I could load up with Cry of Frustration, which may be a better option as it also stops skills and does a small AoE as well. Heck, I may even be as well off to give them Web of Disruption if they'll use it!)