Generally its nearly always the fitting of the boots - I'm guessing you've just got a little bit too much lift in your heels, in which case they may well be just a little bit too big. This could posibly be sorted by:
A - performance footbeds to hold your heel in place.
B - a volume adjustor to sit underneath your existing footbed of 2 - 5mm to decrease that space
C - different lacing techniques
D - slightly thicker socks (maybe - this isn't ideal especially as the weatherss warming up)