Generally if you are weak on the bottom there are 2 main things you can do:
1) Include some DE work so you are used to exploding out of the bottom
2) Strengthen your lats as they are heavily involved in the first part of the contraction.
Something else that helps, is bringing the bar low on the chest whichs helps you keep your elbows tucked and lets you use your lats.
Check out this article and video by Dave Tate: