If she manages 6 inches, it seems to me that she is willing to try really hard to deepthroat you balls deep. That makes you a lucky man !
Maybe she is willing to try harder still. She might not agree with the suggestions I'm going to make, but I'll make them anyway.
Does she allow you to fuck her throat? If so, would she allow you to try to be rougher and go deeper? Would she allow you to continue when it gets too much for her and she's trying to make you stop?
If she is willing to go even further than that, maybe you can tie her hands behind your back so there's absolutely no stopping you. If that doesn't get it balls deep, at least you'll have a lot of fun trying.