Three possibilities based on everything you've listed:
1. Some sort of physical resistance to the motion of the flippers. You'd want to check for proper spacing between the flipper bat and the bushing, as well as for ease of motion.
2. Improper pulse voltage coming from your Rottendog SSFB. I'm guessing it should be +50.0 VDC for the pulse since that's what it is on my R&B machine.
3. Worn out coils. Replace with new ones if everything else checks out.