Hi, and welcome to APUG.

You may have a problem with the PC connection itself, or with the flash synch mechanism.

With respect to the connection, they can get bent. They can also corrode, or get dirty. All these conditions can result in poor contact with the cord.

I have and use a "PC Conditioner" that I bought from a company called Paramount Cords. It allows me to re-form the connectors into the right shape, and helps clean them as well. Most camera repair people will be able to check and repair problems like this even if they don't otherwise work on your camera.