Only half of the light set is working

I pulled every bulb in the non-lit section and tested it on my light tester, every bulb worked. Still could not get the set to fully light. I will get a new set of lights, and test each bulb in the good set to eliminate problems. There are molded plastic gizmos installed on the electrical wire. I think they may be resistors??? I went with LED to get away from lights sets that didn't work, and now I have a more expensive non-working set. Any answers would be great.