Light Failure

Wondering if anyone has any idea as to this.

My headlight comes on when the key is turned(normal) but every now and then when bike is started, the light fails to come back on.
Bike went in for service recently and this was picked up on the diagnostics but no fault was found . As I have said previously, this doesn't happen all the time. After riding a distance and stopping again it seems to be working perfectly once again
Any suggestions as to cause?