According to the instalation manuals on the company's site, the meter only has a sensing input on the plus side and apparently shares the minus side voltage sensing with either the negative power wire or one of the current sensing wires.
In the absense of a separate input, and with the shunt in the negative power cable, it SHOULD be using the I+ sensing wire for the voltage sensing.
Two things to try:
1) Clean up and shorten as much as practical the connections between battery minus and the shunt, and between battery plus and the point where V+ sense is connected. Also, if your bank has batteries wired in series, clean up all the jumpers.
2) Parallel one or more of your spare wires with the one for:
- negative
- I+
- I-
If any of these improves the situation, add as many other spares as you have to it.