Question: If you are weak in the bottom of the squat what do you do to fix it?
Answer: Being weak at the bottom of the squat (or right after a good bounce from the bottom, several inches above) is normal. Why? Because this is the strength curve; the weight is HEAVIER at the bottom than the top. So the first answer is simple: Get Stronger. No one wants this to be the answer because it can never be that simple or that personal; it's taken as a personal attack on their work ethic AND their strength level. Obviously, it's not the former and mostly the latter.
Whether or not you choose to believe this is your business; you can either go through life scouring forums for a non-personal reason why you aren't stronger (usually involves your shoe choice or whatever the new wave of bullshit that is being spread on the internet). However, if you want to get stronger and get serious, you have to be horribly honest with yourself.
Many years ago, about a year after finishing football, I began (what I thought to be) powerlifting. My squat and deadlift progressed nicely but my bench press continued to exist in a sewer. At this point in my life I was buying into the "Weak Point" and the "Perfect Technique" theories that still exist. Namely, work your weak points (not your strong points) and if you miss a lift, it's probably because of some technique flaw.
What a bunch of horseshit.
During that time I was working at the University of Kentucky and the football team maxed out on Power Cleans. An All-Everything Defensive Lineman (who later played quite a bit in the NFL) stepped up to a 375lbs barbell. He bent over, and did a short reverse curl while still in the bottom position.
"Just gotta feel the weight first." Yes, he reversed curled 375lbs while squatting down.
Then he grabbed the bar and with ZERO form and little effort, cleaned the bar to his shoulders. Of course, by Internet Expert Standards (I.E.S.), this was THE WORST THING IN THE WORLD. He didn't extend the hips. He didn't use a double knee bend. His elbows...ugh. Didn't even shoot through at the top to make a strong rack position. He picked up the barbell like a Neanderthal and put it on his shoulders. THE ABSOLUTE HORROR.
He was just insanely strong. Like Lenny strong.
And then it dawned on me that all this dancing around with Weak Points and Technical Engineering of every lift was just a limp-wristed mask to hide behind weaknesses. You either get strong or make excuses. Could it be your thoracic cavity is dormant due to excess lumbago curvature? Possibly. Or it could be that you are a huge weakling that needs to be honest with yourself.
It was one of the best and most brutal lessons I've learned. Honesty is not for everyone. Obviously, there may be other reasons you suck at the bottom of the squat but getting stronger is almost always the honest answer.