It seems like one of the power coils were inverted. Let's see: This generator has two coils, each of them produces 120 volts. You can connect them in series ( as from factory), being the junction point the neutral. From this point to any of the 2 terminals will obtain 120 volts, and from extreme to extreme (end to end) 120 + 120 = 240 volts. But if you invert any one of the coils, ( phase inverting) you will obtain exactly what you are measuring 120 volts between neutral and each extreme but 0 volts between extremes because they are in phase. It means that at any instant the voltage in both terminals will be exactly at the same level, giving as result a difference of 0 volts between both terminals.
So the solution is very simple, if this is the case, just invert One of the coil terminals coming from the stator to the power connector, in this way:
Before: a bnc d After: b anc d
Being a and b terminals, of coil 1 and c and d, terminals of coil 2, n is neutral.
I hope this help you.