So, I placed a  build_light in my map, and it  works fine as an omni, but when I change it to primary_spot, it no longer works. also, I have no idea how reflection probes work in the new radiant. HELP??

I personally have never used the build_light.

I did use the unsorted_light as a volumetric light earlier and it worked fine as a spot light.

As for reflection probes I would assume they work the same as in waw, if not just take a look at one of the premade maps and check how they do it.