The problem with squats when to comes to glute-building is that the top quarter of the range of motion (when you go into hip extension) is basically devoid of any resistance for the glutes.

This is why glute activation measures are consistently higher for people in the hip thrust than the squat. The glutes are maximally loaded in the shortened position in the hip thrust.

Here's an exercise to fill the gap... and fill out your flat butt:

The Banded Landmine Deadlift

What I like about the banded landmine deadlift is that you're using a somewhat modified sumo deadlift stance, which means you'll naturally be executing with the hips externally rotated. You can't get the glutes fully shortened without external hip rotation.

Since we're adding accommodating resistance (bands), we're making sure there's no "dead area" within the range of motion – there's no area during hip extension in which the glutes don't feel resistance.

This is a great exercise for chasing some rep PR's. I can promise you that knocking out a brutally hard set of 15-20 reps on these will smoke the entire lower body, especially your money-maker.

Related:  Real Men Train Their Glutes

Related:  The Flat Butt Fix