Back Legs Problem

Hello,

Every time I try to get Hexy to walk, the back two legs sort of give out and start dragging. This slowly pulls the rest of Hexy down until it is left floundering around on the ground.
Is there any way to prevent this from happening?

Also, in past I have managed to get Hexy to walk, but it always veers to the left. How would I work around this?

Thanks.