Some hips are stopped by bone and stretching will not move them

A squat depth or a rotation limit can be set by the shape of the socket itself, which no stretching will change.

Two athletes cannot get their hips below parallel. One is restricted by soft tissue and will improve within a month of loaded end range work. The other has a femoral neck that meets the rim of the socket at ninety degrees of flexion, and will be exactly where they are now in ten years. Told to keep stretching, that second athlete will grind cartilage for a season instead.

Hip shape varies enormously between individuals. Socket depth, socket orientation, the angle at which the femoral neck sits relative to the shaft, and the presence of extra bone at the head and neck junction all differ, and much of the variation is set during skeletal growth. Some of it appears to be shaped by loading during adolescence, which is why certain sports carry noticeably different hip morphology in their populations.

When bone meets bone, the athlete does not feel a stretch. That is the clinical tell worth knowing. Soft tissue restriction produces tension felt in a broad area and it eases as the tissue creeps. A bony block produces a hard, abrupt end point, often with pinching felt at the front of the joint or deep in the groin, and it does not creep at all. Pushing into it repeatedly loads the labrum and the cartilage at the rim, which are the tissues that fail in that scenario.

The compensations follow. A hip that cannot flex any further will get the last few degrees by tilting the pelvis backward, and the lumbar spine rounds under load to allow it. That is the mechanical origin of a great deal of low back pain in lifters who have been told their problem is core stability. It is not a strength issue. It is a range issue, being solved by the only joint willing to volunteer.

My view is that squat depth should be individually determined and then left alone, and that a large fraction of the coaching cues about hitting depth are asking bodies to do something their skeletons declined.

You can test this crudely. Lie the athlete down, flex the hip, and add a little rotation.

If the block moves when you change the angle, it is tissue. If it sits in exactly the same place every time, you are feeling the shape of the person.