Welcome to the forum.
A P90 uses 2 alnico bar magnets on the bottom of the pickup.
Both bars have to be oniented the same polarity.
Usually N/N, is how they are oriented.
S/S would also work.
With 2 pickups one N/N, and one S/S would give RWRP with both pickups in use.
You don't want them S/N, or N/S.
That would reduce the magnetic gauss, and make a weaker pickup.
I would start by checking to see that the magnets are oriented correctly.