Struggling to squat deep usually is not a strength problem; it is a joint that will not let you get there. Ankles, hips and upper back each limit depth in their own way. Test first, then drill the actual limit.
Test before you drill
Do a slow bodyweight squat and watch three things. Heels lifting off the floor points to tight ankles. The chest collapsing forward points to the upper back. A pinch in the front of the hips at depth points to the hips.
Do the test without shoes on a hard floor; cushioned soles hide heel lift and muddy the results. Then hold a door frame and squat down: if you can reach depth with that counterbalance, mobility is the missing piece and the drills below apply. If not, strength and practice are the priority.
Start with the ankles
Kneel with one foot a hand-width from the wall and drive the knee toward the wall without letting the heel lift. You should touch the wall with a little range to spare; if you cannot reach it, the ankle is limiting your squat.
Spend two minutes per side every day on knee-to-wall rocks, plus a calf stretch with the back knee straight and then bent. If one ankle is clearly stiffer, give it an extra minute and lead with it in your warm-ups. Ankles respond better to frequent light work than to long occasional sessions.
Open the hips where they are tight
Do 90/90 rotations and a supported deep squat hold, gripping a rack, door frame or a light weight, for 30 to 60 seconds daily. Breathe slowly through the holds; holding your breath makes everything feel tighter than it is. Let gravity pull you down while you keep the chest tall.
Add slow cossack squats from side to side, staying as deep as you can control. The goal is range you can move in, not a stretch you can survive.
Practice the bottom position itself
Hold a goblet squat in the bottom for 20 to 30 seconds with your elbows inside your knees, gently pressing them out. The weight acts as a counterbalance, which is why this position unlocks depth for so many people.
If your heels need support, a thin book or a pair of small plates under them is fair while you build range; it is a tool, not a crutch. Sit in a deep squat for a minute or two every day, TV time counts. Frequency beats intensity here: five short exposures a week outperform one long session.
Warm up into depth, every session
Start each leg session with bodyweight squats to your best depth, then goblet squats slightly deeper, then your working sets. Depth practiced cold never sticks.
Re-run the door frame test every couple of weeks; objective checks beat guessing whether it feels better. Expect progress over weeks, not days, and never push through sharp pain or a hip pinch. Log your best comfortable depth in Bulky; watching the bottom get easier is the motivation that keeps the drills going.