Hi, I'll be glad to assist you. When one of the lights burn out the circuit is broken and that section won't light. Replacement of the lamps is the only way to correct the problem. Even if you found the one that is causing the problem, another will probably fail shortly after and you'll be back where you started. Replace all the lights on the affected string and it will last a couple of years.
So on the strands that do work, there are a few bulbs burnt out on those strands. Do you know why those strands would still be working?