The twin contacts in bulb holder sometimes go bad contact, in that. there is not enough constant tension against the bottom contacts of the bulb. On one of my previous motorcycles I had a similar type of problem. Difficult to increase the tension of the contacts inside the bulb holder, to ensure contact to the bottom of the bulb. To solve the problem, I added a little bit more solder to the bulb connectors, to give it a taller / longer profile, thus allowing for a better contact. Worked like a charm.