Before attempting DIY maintenance, ensure proper safety precautions to prevent electrical shocks
Interconnecting lights are not working
There is a small key in the extendable adaptor to ensure proper alignment ie. ensure the lights are correctly plugged into each other.
Some strands are not lighting up
It means that one wire has probably pulled out (disconnected) in the dead section.
Look for a loose wire, or a discoloured globe.
Remove the bulb completely, and /or reconnect the wire - Replace the wire into the sleeve (and use super-glue), or remove the bulb completely and connect the wires to allow the current to continue to flow.