make sure the ends of the bulbs are in the sockets and they meet up with the end of the bulbs. I had this happen to a fixture once not long ago and found that the bulb and the socket were not together close enough for the pins on the bulb to make contact with the contact in the sockets. pushed then together by holding the bulb and gently pushing the socket to wards the bulb. I say gently because you do not want to break anything the pins on the bulbs in allot of cases or held on place by epoxy some have a aluminum cap around the end and if the moves could cause a short. check that also.

Bill