Hello. Installed two 50 runs of Led rope lighting today. One side is bright and the other is dim. The dim side runs through a GFI plug breaker and they both have photo cells hooked up to them.
Make sure the photocells aren't being illuminated by the lighting itself.
If non-polarized try reversing the plug on the DIM string.
I've had a few LED Xmas strings exhibit that behavior until
the plug reversal.