You may be restricted by your flexibility. Be careful that by trying to go too low you don’t round your back and loose your form. I would think that thighs just bellow parallel will be low enough to work your glutes well.
hmmm unitl recent years I thought parrallel is when the top of my quads was parallel with the floor, not I just go lower cause for me is just feels better plus it really helps the glutes and back out a lot going so deep.
Depends on what you're doing. For power lifting the hips just need to be below the knees. Any lower it's considered to be a waste of effort. In Olympic lifting, you need ATF to be able to catch the big weights.
As has been said, the lower you go, your hips/glute/hams are recruited more.
I go down until my calves and hams seem to fuse together as one. For some reason that is the only way I feel strong on squats. When I try to stop just at parallel it has always felt odd for me.
In order to achieve this depth work on hip flexibility,
I recommend at least a shoulder width stance. . with some box work. I find that this helps with depth. Also, if you are feeling too much stress in the knee while squatting re-evaluate your form. The squat should be about hip and upper quads.