Is your battery fully charged? Low battery will make em run slow or not at all. Maybe you need a more powerful battery? What kind of servo arm are you using? The one that came with the servo? Can be another problem. Just food for thought.
Nearly my entire fleet consists of retracts. One other goodie is the type of wire used for the retracts. The cheap crap that comes with it can "Bow" out when pressure is applied. I replaced all mine with a 4-40 rod.

I missed the servo your using, the 77bb is not a retract servo and willnot properly work. Get the 75bb