A: First, verify that it is the jack and not the cable. If it is the guitar jack, there are several things that could cause this. The most common two are:
1. The jack is dirty or corroded. Some contact cleaner may help. Or, if the jack is corroded, a wire brush may be required to clean off the corrosion.
2. If the jack isn’t making firm contact, the strip of metal that pushes against the tip of the cable plug may need to be bent a small amount to ensure a good connection. A gentle bend of the metal strip may solve the problem.
Either of these are easy fixes, but if you doubt your ability to open the guitar up to access the jack or are not comfortable working on your guitar, then any tech should be able to fix the problem quickly and easily.