The boots may not be the wrong size, they may simply be the wrong boot for your feet. I have a narrow heel and I get heel lift in 95% of snowboard boots. You just gotta keep trying until you find a pair that works. Do you have narrow feet? Have you ever had heel lift issues in hiking boots, etc? If so, I suggest checking out the Salomon F series boots. If not, you may just have a pair of boots that run on the wide side. What boots do you have?
I'll second checking out the Salomon F boots. Even with my friends F20s beings 1 full size too large they held my narrow heel.