I would suggest having the magnets checked to see if they are magnetized and calibrated right. There are many things that can demagnetize the magnets in your pickups and care should be taken not to cause any electrical interference with them. Often, I’ve seen pickups taken apart or out of the instrument and the magnets were put back in wrong, especially with Gibson P-90 single coil pickups that use two magnets. The pickup will read but if the magnets are in wrong, there will be minimal output. Also make sure you replaced the magnet in a humbucker in case you were working on it. Make sure you are using strings made for electric guitar and not acoustic guitar strings. Pickups rely on the movement of the magnetic field from the ferrous strings. Some acoustic strings will work, but you are only picking up the ferrous spring steel inner core. Make sure you have the hookup wires connected to the right connections on the switch.