I've tried creating a custom map at least 10 times over. AKA, in my case, trying several times, radiant bugging out, or freezing, and me ending up having to delete all the files (around 3gb~ of files I think). I've done this multiple times over.
Most of the times, things won't show up for me to place, such as volume areas, or player spawns, or zombie models.
I deleted it again last month, and haven't tried re-installing since then. If anybody could help me with this, I may be willing to try again.