With dense mixes, it can be tough to get a lead vocal or lead instrument to sit well and be audible. There are two tricks you can use to fix this problem:
1. Carve out frequencies in the backing tracks that will allow the lead to be heard. You may also benefit from boosting the same frequencies on the lead.
2. Pan the backing tracks away from the center of the stereo field. This will leave a spatial hole for the lead to sit in.