1st & 3rd string of Christmas lights in a chain work but 2nd doesn't

I have a fake Christmas tree that came pre-strung with Christmas lights that my family has been using for several years. The tree comes in 3 sections (bottom, middle, and top) with 2-4 strings of lights wrapped on each section.

This year, the bottom and top section’s strings light up, as well as the middle string of 3 in the middle section, but the 1st and 3rd string of the middle section won’t light up. I can see that the current is running just fine throughout the tree as the bottom and top lights work. Any ideas why 2 of the middle string of lights aren’t working? And how to fix this?

Thank you! Please help my family save Christmas!