I have narrow heels and had the same problem with Nikes. Especially because they seem to pack out a lot. J-bars, molding, and so on... Nothing helped. Tried Salomon Dialogues afterwards which was a bit better but still no cigar. After a few recommendations by boot fitters I found the Nitro Team TLS. No heel slipping whatsoever because of a very articulated heel cuff. I don't know if you want to buy new boots but if you do, definitely try Nitros and maybe compare them to Salomons.