Here's some reasons for weak flippers on electromechanicals.
Your voltage could be low. Check the voltage with a meter. If low check
the transformer tap and see if the line is connected to the low line rather
than the high.
Even if your voltage is high enough your amps could be limited due to a bad
connection with your fuses. See if one is getting hot. If so, clean the
contact areas on both the fuse block and the fuse itself.
Check to see that the flipper mechanism is not physically binding, or that
the linkages are too loose. This many require tightening some things up, or
getting replacements that are not worn.
See that the flipper return springs are not wound too tight. This is a
common problem on Gottlieb flippers. The springs should be just tight
enough to return the flippers to resting position when the playfield is
down. When the playfield is up, it is okay to have them not return all the
way. You may need to change what screw with which the springs are fastened
to the flipper bushing.
Check the end-of-stroke, EOS, switch that it is clean. It is okay to file this.
Be sure that it is gapped properly and opens at least 1/16" when the flipper
is fully engaged.
You might also want to check the flipper button switches themselves to see
that they are clean also.
Also check the coil itself to see that the heavy gauge (power) winding is
engaging. If the wire or solder joint is broken then you may only be
getting power through the hold (weak, thin gauge) winding. This may also
happen if the EOS switch is always open.