A friend of mines guitar has a similar problem. Open up the cavity and unscrew the jack so u can take it out. Take a look at the wires connecting to it and make sure that they arent fraying or coming unsoldered. However, sometimes the jack itself could just be busted in which case it needs to be replaced.

What kind of bass are you using? If I know what kind of bass you are playing I could recommend the kind of pickups you would need to get the tone you are after.