Very common problem. Most of the time it will be a physical issue. With the game off and the playfield raised, you need to check a few things ;
Move the kicker by hand. Feel for resistance.
Remove the coil bracket. Feel for resistance. If any part of the assembly was previously lubricated, you need to remove the assy, disassemble, degrease, clean and then re assemble.
Check the alignment of the bracket in relation to the coil bracket. The plunger must move freely, and you can pivot either the coil bracket OR the assembly itself.
The assembly should move easily by now.
If not, try another spring - one that has a higher tension. I've found this to work when all else fails..
