Aerospace Logic Volt/Ammeter

Intermittent problem with an Aerospace Logic VA-100 Volt & Amp
indicator.
Volt side works at all times and is accurate, amp side works properly
occasionally but more often than not displays -0.0 amps. When it does
work it indicates charges and discharges properly.

Unit supposedly shipped with a 60A/50mV shunt. Aircraft has a 60A
alternator. Unit measures mV change across shunt.

Red side of shunt from battery / Black side to buss
Twisted pair from indicator, red to red side of shunt, black to black
side of shunt.
Black wire from indicator to ground
White wire from indicator to rheostat for back lighting
Blue wire for 28v system not used.

Instrument set up for 60A, high and low limits within the proper
range.

When amps fail to indicate, battery only will not show a discharge and
engine running w/alternator output will not show a charge. Both cases
indicate -0.0. No change with a change in load, it will continue to
indicate -0.0.

Any experience or thoughts?

Thanks,
Jim