Check the knife bushing at the wobble drive arm. If its worn out and the rubber isolator is gone, knife motion is lost. Also check the wobble drive belt tension. If its slipping, the knife will stall. It could also be binding up because a blade is colliding with a guard. Make sure all the knives are contained within the guards. Lastly, use a hammer to pound the hold-down clips into position. They should constrain the knives down to the ledger plates on the guards.
Lay the cutter bar down on a nice flat surface (like a concrete floor). The tips of the guards should all be in a flat line. If a few are bent up or downward, slip a long pipe (5 - 6') over the guard tooth and bend them back into line. You are really bending the mower floor plate, not the guard.