I'll go against the grain here and say you just need to get stronger period. Assuming it's not a technique problem, there will always be a sticking point in a raw SQ. It sounds like you slow down after using a stretch reflex to drive you out of the hole.
This is pretty typical and doesn't necessarily signify any particular weakness. The only thing you might consider are some pause squats to strengthen the bottom of the lift so you don't rely too heavily on stretch reflex to get you out of the hole. If you are able to drive harder out of the hole and add the stretch reflex on top of it, you are less likely to slow down as much at that sticking point.
Edit: Missed that this was a box SQ. The above would apply in a free SQ. I would tend to agree with the reply about rolling forward off the box if this is your sticking point on a box SQ.