When I had a couple of items in the pool that were set to zero it produced this error.
[Error] Pool Boss has not initialized (does so in Awake event and may take additional frames if you configured it that way) and is not ready to be used yet. Check that PoolBoss.IsReady returns true before calling other methods.
None of my spawns would initiate.
Once I change them from zero or deleted them the error went away and the pooling worked correctly.
I had to guess at what issue was producing this error.