A coil is weak, dead, buzzes, gets hot, or keeps blowing its fuse or driver transistor.

Checking a coil against its chart resistance

Cause: An open winding (broken wire or lug joint), shorted turns from overheating, a wrong coil fitted earlier, or a failed diode across the coil.

Checks

Find the number printed on the wrapper and look up its resistance in the chart; for a flipper coil note both values.
With power off, lift one wire from the coil (or one leg of its diode) so other circuits do not affect the reading.
Touch the meter leads together on the lowest ohms range and subtract that reading; it matters for coils of only a few ohms.
Measure across the coil lugs. For a flipper, measure the power winding and the hold winding separately, using the lug the chart pairs with each.
A reading within about 10 percent of the chart is fine. An open circuit means a broken winding or lug joint; a reading well below the chart points to shorted turns or a burnt wrapper.

Resolder a broken lead at the lug if the break is there; otherwise replace the coil.
Replace with the same number, or any coil with the same gauge, turns and bobbin size.
If the coil read low or looked burnt, check its driver transistor or EOS switch before powering up.

Turn the game off and unplug it before touching coil lugs; old EM and SS games have line voltage nearby.

Source: Master Cross Reference Chart of Coils for Coin Operated Games (last edited July 25, 2016)