If it is the sub bass ur thinkin off.... make sure u got a full range monitoring system so u can hear it properly. Ive been mixin on speakers that range from 55hz to 22khz and could never quite get that sub workin how i liked.

Now when i work on sub, i use my headphones that have the full 20hz - 22khz frequency range and it has helped me a lot. This could b shite advice though :)