I would think to check the 5 th coin multiplier unit in the head.
i.e. the X5 unit ... the one that looks like a pinball machine score reel.
if it's not stepping as coins are ejected, either the coil is bad, there's a problem with wiper connections on the odds unit disc, or a wire is broken off.
if the X5 unit is stepping but the payout unit isn't, check the wipers/contact plate on the X5 unit itself.
tmi
----
the idea behind the XY (X2-X5) units is none or one gets selected by the odds unit, and during payout the selected one steps for every coin ejected. However, it only passes the pulse to the payout unit step-up coil every Y steps, resulting in multiplied payout.
the other ways bally did payout multiplication on some models was via cams with different number of lobes or by resetting the payout unit and paying again as many times as needed.