Hi I'm hoping someone can help me.

My entire left side indicator lights do not flash. When i put the indicator on, the lights inside flash normally, but the lights on the outside does not flash. It stays on for about 15 seconds after I've turned the indicator off.
The problem occurs randomly, and the 2 mechanics Ive taken to has told me they cant find the cause of the problem as they cant see it first hand.

Any suggestions would be appreciated.