Old.Fart

Hi i'm new and have a small problem with my headlights. I've explained the problem in the electrical section/ forum thing.

Fitted new headlight bulbs, passenger side has no problems, drivers side bulb fits ok and light works ok. However after about an hour or two the light on the drivers side goes off and leaves only side lights (no dipped beam). nay suggestions as to what the problem may be and how i can go about solving the problem.