[1.16] Getting the entity to spawn

I have a functional creature-entity (I can get it when i use the spawn egg). That was too frustrating but I'll try not to vent.

Now i need those things to spawn in swamps.

What i have:

1) in FMLCommonSetupEvent i have just one line:

EntitySpawnPlacementRegistry.register(RegistrationManager.TESTSLIME.get(), EntitySpawnPlacementRegistry.PlacementType.ON_GROUND, Heightmap.Type.MOTION_BLOCKING_NO_LEAVES, TestSlimeEntity::checkTestSlimeSpawnRules);

event fires, but checkTestSlimeSpawnRules is never called. i don't know why.

2) in BiomeLoadingEvent just this:

if (event.getCategory() == Biome.Category.SWAMP)
{
    event.getSpawns().getSpawner(EntityClassification.CREATURE).add(new MobSpawnInfo.Spawners(RegistrationManager.TESTSLIME.get(), 8, 1, 4));
}

event fires a couple of times (swamp i'm in is likely composed of few small biomes; it is cut up by other biomes).

3) in entity registration, it is classified as EntityClassification.CREATURE.

any idea why they won't spawn?