Not sure if it coincided with natural population swings, but I have noticed this twice.
Had a bunch in my 75, added both a potter's dwarf and an C. deborai within weeks didn't see any of them on the glass or rocks. After a screw up on my part, I lost both of those angels. Within a month, the tank was covered in them.
When I added an C. venustus and C. acanthops they disappeared again within weeks of being added.