There are two possibilities as I see it.
One, you just aren't getting tight enough. Easy fix, get tighter. Problem solved. However, I'm not entirely sure that's the issue.
Two, you aren't strong enough in the trunk to keep your torso at the angle you need to be at to hit depth. You lean more and more as you go down, so if you went any deeper you'd get pinned.
The fix for that is to get your back and abs stronger. Getting tighter will help, of course, but until you're strong enough to hold that position it won't entirely fix the problem.
Back extensions for high reps are good, so are good mornings and reverse hypers. Also barbell rows. An rollers for your abs and also suitcase holds or carries.