If a specific section of the lights isn't working, there might be a bad bulb, or a bad connection between the bulb and the socket.

Gently grasp each bulb, and pull away from the socket. Inspect it and ensure that the two bulb copper leads are in their proper location, and not twisted or missing.

Continue with each non-functional bulb in the chain, up until you find the culprit(s). Replace the bulbs as necessary.