maybe it's the drums. In a mix, when you hear the guitars you actually hear what guitars sound related to other instruments. Instead of working on the guitars alone, concentrate on the whole mix, try to find a balance with each instrument. Great mixes consist of great individual instruments (both tone wise and playing wise - you don't say 'this is a great mix, but the kick does teh suck!') but just because you have great individual tones of drums, bass, guitars, that doesn't automatically mean that you'll have a great mix in the end.