I had the same problem. The metal plates fit fine on the lights.. but once the plates were mounted to the fiberglass tips, the lights wouldn't want to slide onto the plates.
I believe i sanded down the fiberglass just a hair so that the mounts and the lights had a nice flat surface. But before you do that, on one of my lights, I had luck with putting a little grease on the metal plate's 3 prongs that latch into the light. The interference was so minor that with the grease, I was able to manhandle it on.
Rob Carsey, North Brunswick NJ
RV-12iS (N713) / Completed 12/2020
ASEL, Glider, AGI/IGI, LSRM-I, FCC GROL