Help troubleshooting light fixture

I received a european light fixture that the seller converted from european E14 sockets to E12 US sockets. There are 5 sockets on the fixture. I bought new bulbs and was able to get 1 socket to light. I pulled all the tabs out on the sockets, same result. I tested with the voltmeter and all sockets are getting the same voltage reading as the main wire coming through the ceiling. I then took the new bulbs and tested them on a different fixture, and all worked.

Lastly, and this is the confusing bit, I took a different light bulb from an existing fixture and was able to get it to light every single socket, BUT, then I tried 20 other bulbs and no other bulbs are able to light any of those 4 sockets I'm having trouble with.

Can somebody please tell me how to troubleshoot this?
Thank you.