- Wires out of phase. When both pickups are selected, they partially cancel each other, greatly reducing the volume.
- Magnets reversed. Multi-pickup installations must have the magnetic poles alternating North-South-North-South, or vice-versa. If the pups are positioned with two N or two S poles near each other, the magnetic field doesn't interact with the strings properly, resulting in thin, weak sound.
- Poor ground connections. Even big-name guitar companies go cheap & dirty when they install the controls. Its never a good idea to use the backshell of a control pot as your ground connection. Check out the >wiring< section at GuitarNuts for pointers on how to do it right.
- Damaged parts. Did you spend a lot of money on new pups, but re-use the old pots and jack? Good pots and jacks cost about $5 each. Flaky used ones cost major downtime and frustration.
Good luck! :)