you can compare the coil resistance vs what's listed online for that number. Also make sure the contacts are well adjusted. Could also technically be a bad transistor on the board, but it's unlikely

Check the pivot hole of the A-12664 slingshot kicker arm itself. Too much slop in that hole will cause "lost motion" which will make the kick weaker. Also, the hole in the plastic link on the slingshot plunger might be egg shaped, out of round, or cracked. That can also cause weakness in the kick.

Rubber ring too small?

PS: Coils don't 'weaken' over time. Outside forces cause them to fail/degrade in different ways. Weak action is the symptom of several possible causes.