Assuming that you're listening to the instrument in mono (either at the power supply output or 1/4 inch in mono mode):
When you're in the neck pickup (bass channel) only, the bridge pickup (treble channel) is grounded. So this give you a 6-dB reduction due to the mono mixing resistors which is normal and intended (when one pickup is selected, the opposite channel has to be grounded, otherwise in stereo mode, the lead would be open and you'd get hum and cable microphonics).
So I think that pushing on the pickup selector knob is causing the opposite pickup to be ungrounded (thus making the selected pickup louder than normal. Exercising the switch would be a good thing to do to see if the contact reliability would increase, but it's not usual for this to be needed. The switch may have worn with use, and the contact pressure no longer sufficient for reliable operation. These switches are not openable and are semi-sealed against environmental contamination, so cleaning is not possible.
Until recently, the switch manufacturer had a c-washer on the shaft at the end of the bushing which controlled the shaft position against the bushing, and thus the contact pressure. The end of the bushing may have worn slightly over the years and so a possible approach would be to remove the c-washer and put a thin shim washer under the c-washer to take up the wear. Not easy, but doable. Also the contact wafer is held in these switches by a crimping over of the switch housing, and if that has loosened, then you could manually push the plastic switch wafer into the body and retain it with crimping the housing at that end. But likely replacement is the best option.
