Also make sure you are using a stereo plug.
For those that don't know a stereo jack will have 2 black bands on it where as a mono one will only have one. These days it's less likely to find single band ones on leads made for such things but it can happen.
If it's a stereo jack I'd suggest the same as Gorms did, check your plugs for being seated right because a stereo plug not pushed in can make contact with one side and not the other and still work.
However a seating issue doesn't sound completely right if you reckon the sound equalises when the balance is shifted so I'd be checking the cabling a bit more thoroughly if reseating it doesn't work and it's a stereo plug. At a stretch it might also be a broken wire in the cable, but either way replacement cables are easy to come by to do a test.