I had narrowed my shoe selection down to a few shoes and went to REI yesterday to try them on. I tried the Salomon XA Pros, Brooks Cascadia 6 and 7's, and a variety of Vasque. I found that I have heel slippage in every pair, but only on my left foot. What does this mean and how do I fix it? Any solutions? I really want the XA's! They feel great beyond the heel slip. Suggestions please!