I hadn't seen phantoms at all during my playthrough despite not sleeping for at least 20 nights in a row. I tried walking around outside at night every night, but I still didn't find a single phantom. Then, I made a glass box in the middle of a large plains biome to try to lure them out. I've spent several nights in this glass box staring at the sky, but have not seen a single phantom. 

DoInsomnia is true.

Difficulty is was easy, but I switched it to normal to try to get more phantom spawns. No luck.

Playing on an unmodded 21w11a multiplayer server.

EDIT: Apparently dying resets the insomnia clock.  I had no idea. I finally got them to spawn.