Taken from : https://rec.games.pinball.narkive.com/2SBY4Oiw/tech-sttng-left-flipper-weak-checked-obvious

a.. Rebuild the flippers. Play and wear in the flipper parts is the
primary reason for weak flippers. A mushroomed flipper plunger dragging
against the coil sleeve is a classic cause of weak flippers.
b.. Make sure there is about 1/16" up and down play on the flipper. To
test this, from the top of the playfield, grab the plastic flipper and pull
up. There should be some play. If not, the flipper could be binding on the
nylon playfield insert. This gap is adjustable from under the playfield by
changing the flipper pawl's grip on the flipper shaft.
c.. Make sure the EOS (end of stroke) switch is properly adjusted. On
games with electronic flippers (fliptronics), the EOS switch should close
right at the end of stroke, and not prematurely. If the EOS switch is
misadjusted, this can cause a slightly weaker flipper on old and new WPC
games.
d.. On WPC fliptronics and later games, try cleaning the "U" shaped optics
on the cabinet flipper opto boards. Use a Q-tip and some Windex to clean
them. Also make sure the opto activator bars fully clear the optos when the
cabinet switch is pressed. If one weak flipper still exists, try swapping
the cabinet flipper opto boards (remember, both flipper boards must be
plugged in for this to work!). If the weak flipper problem moves to the
other flipper, the opto board's optic has become faulty, and it will need to
be replaced with a new "U" shaped optic. A marginal cabinet flipper board
optic, even if clean, can cause a weak flipper. Replace if in doubt. Also
check the opto switch with your multimeter. With the game on and your meter
set to DC volts, on the flipper opto board measure the connector pin marked
SW1 and SW2 against ground. A measure of below 0.7 volts (below 1V is OK)
should be shown, with the button pressed. A higher reading means a dirty or
defective opto switch. If cleaning does not remedy the problem, replace the
opto switch. NOTE Later WPC-95 pinballs use a Schmitt Trigger opto switch (3
legs on the receiver, 2 on the transmitter) which eliminates this problem.
The Schmitt trigger optos will not oscillate (turn on and off quickly,
making the flipper weak) when the optics gets dirty. They usually either
work, or don't work.
e.. On WPC fliptronics to WPC-S, replace the U4 and/or U6 LM339 chips on
the Fliptronics board. On WPC-95 games, replace U25 and/or U26 on the CPU
board (since these games don't have fliptronics boards). Although these
don't fail often, then can cause weak flippers. See "WPC Fliptronics Flipper
Optos" in the switch matrix section for more details.
f.. Check the flipper power connections. On WPC fliptronics to WPC-S
games, this is connector J907 and J902 on the fliptronics board. On WPC-95
this is connectors J119 and J120 on the power driver board. Make sure the
solder joints on these board header pins are not cracked, and that the
connector and header pins are in good shape.
g.. On WPC fliptronics to WPC-S games, check the fliptronics board bridge
rectifier (BR1). On non-fliptronics games, check the bridge and capacitor
that supplies voltage for all coils (BR3 and C8). An open diode in the
bridge rectifier that supplies power to the flippers can cause weak
flippers. A fatigued or cracked solder joint on this bridge (or its
associated capacitor) can do that too. Soldering jumper wires from the
bridge to its associated capacitor is a good idea. This is rare, but does
happen. This problem will effect BOTH flippers equally. See the section,
Testing Bridge Rectifiers for more information.

And further down the post , Sir LTG writes this : Besides the obvious, be sure the brackets holding the coil aren't leaning in
or out. LTG

And then the OP answers this : Lloyd Olson nailed it... one of the brackets was slightly off axis...
I loosened up the front bracket (the one without the coil stop) and
WOW! What a difference!

Hope this helps!