We don’t necessarily deal in Christmas lights but I would be more than happy to brainstorm with you!
There can be a number of things that might cause Christmas lights to get messed up. There are so many different ones out there that are all made in so many different ways that there isn’t really a one size fits all approach, so to speak.
A few ideas:
Some of them short the bulbs when the filament fails, which allows the other bulbs to still work but that socket no longer will.
It’s possible that the replacement bulbs you tried are not compatible?
The socket itself could just be bad (Are these lights new?). If they are older lights, some of the wiring could be damaged from being moved in and out of storage and coiled, etc.
It is also possible that heat from the lights may have damaged/warped the socket.
Thankfully, most Christmas Lights are pretty inexpensive in my experience, so you may be better off just buying some new ones instead of tinkering if it comes down to that.
Hey yeah so these are some older lights, at least they are ones that I have used before for a few years now. I wanted to see if there was a way to try fixing them rather than having to throw them away and/or get new ones. Just chalk that one up to me being a cheapskate I guess, lol!!