Also make sure they are the correct coils, I have bought machines before where the previous owner put underpowered coils in the flippers. Right flipper is a 22-900 and left is 22-1080.
Check the coil sleeve, make sure it isn't too worn.
Check to see if the coil has expanded and the sleeve is now too tight in the coil.
Clean the sleeve and the plunger make sure the plunger goes into the sleeve smoothly.
Some people replace the flipper coils on that machine with stronger alternatives, but my BF runs real well with the stock sizes