Are you in Journey Mode? If so make sure you don’t have spawnrates turned off.
Make sure you aren’t near a town or any NPCs which will greatly hinder how much enemies will spawn.
Make sure the Hallow Biome is big enough to change the music and background to Hallow. You may have to get farther from the mushroom biome to tell. It might have to be bigger, but IIRC it doesn’t take much pearlstone.
Finally, keeping most of the Pearlstone offscreen (but still being in the biome) will greatly increase how many hallowed enemies you get because they spawn on pearlstone but no enemies can spawn onscreen.
You can also get souls from non-Hallowed enemies in the Underground Hallow, don’t stress if you’re not getting many Hallowed enemies as long as you’re getting souls to drop.