I'll admit...I know nothing about lighting or lightbulbs. I have a 24' set of 12 outdoor string lights that are only about 2 years old. All but 4 of the incandescent bulbs burned out already. I replaced with a set of LED bulbs but only 2 of the 12 worked. I tried switching around bulbs but only the same two sockets worked. The odd part is that none of the two that worked are the same as the four sockets that worked with incandescent bulbs. Any ideas what my issue is?

Lacking all the relevant info, here's a guess -

Your incandescents were low voltage bulbs (12 or 24 V) wired is series to use the 120 V supply.   You replaced them with 120 V LED bulbs.

Do you have a DMM or DVM?  Check the socket voltages after lighting as many as will light up.  To use a 120 V bulb, you need 120V at each socket.

@Trollslayer

I tested the sockets and wasn't getting 120 consistently. I took the whole strand back and had them replaced. Now they work.

Thanks for the assist!

Corroded sockets. LEDs probably need 120volt to even work. The old incandescent can run at any voltage  really.

Just a guess.

I have seen problems with non dimmable bulbs . Also switching a 3 way you can only use one dimmer.