It's often one of the bridge volume (or tone) lugs touching the copper shielding and thus grounding your signal when it's selective problem like this. Sometimes even a stray wire strand or solder blob can do it.
Just check that none of the pot lugs are brushing the shielding and/or the underside of the lug board isn't being squeezed down onto the copper (you can use some electrical tape to insulate the underside of the lugs if needed).