Then they might be missing the whole point of the ability in the first place: the reason the power is a -0 refresh ability is that it's also a curse. That's the way it's supposed to work. If it were meant to work as an infallible predictive talent, it'd cost more... perhaps a
lot more.
They may
want to believe. They may not understand why they don't. They don't even have to
disbelieve, they're simply pressured by that curse into justifying why they won't act on the information. They may instead not believe it's important enough, compared to everything else they're dealing with. You're saying that they have a hard time RPing disbelief in something when they patently
do, as open-minded and at least clued-in PCs. That's logical. As a fellow GM, I'm just saying that this
is an RP challenge, for everyone involved, to try and roll with something like this. It also somewhat "cheapens" the choice of the character with the ability, if part of what they wanted to RP was having to struggle with that disbelief.
You can even leave the compel out of it. They can choose to act, or not, regardless of what they do or don't believe. Make it a simple RP situation, and leave it at that. Just challenge them (as above) to RP from the standpoint of disbelief.
It should go without saying (but I'll say it anyway
) that this is just my take on the situation. Ultimately, the entire group should be on the same page and reasonably happy with the final decision.
Afterthought: it also occurs to me that, when Harry encounters Lydia, he doesn't believe what she's telling him at face value... and he's a full wizard who knows about Cassandra's Tears. My take on that entire exchange is that, in the end, he doesn't necessarily believe what she's telling him, but he
does believe that she's terrified out of her mind and in need of help, and decides to act on that basis. He RPed doing something to help, even while not fully accepting the nature of the problem.