What could cause bumps on the head that leave a patch of hairless skin?

Pilar cysts: Sounds like a pilar or sebaceous cyst. These commonly occur in the scalp and can be tiny or grow quite big in size causing a large bump without hair to form. These usually need to be surgically removed by a dermatologist or dermatologic surgeon.

Possible tinea: Tinea capitis, a fungal infection of the scalp can cause bumps followed by circumscribed area of hair loss.