The same thing happened to me a while back w one of my sidi's but I have no idea why It took me a 1/2hr to get the buckle undone

I think perhaps if you press gently on the ratchet it might work? I think that's what I eventually did..

Sorry I can't be of more help but I just remember trying to get my right shoe undone after mtn biking one day & it just didn't happen!