The feeling of numbness in the back of the head can be very unpleasant. It is a well-known complication of an operation to release a nerve. Some patients also report scalp hypersensitivity or scalp pain instead of numbness.
Loss of sensation as described can be permanent. Return of nerve function can take up to 18 months. If you don't have normal sensation in another year, you aren't likely to get it later.
If this symptom gets worse or you develop new problems, talk to your doctor. There may be treatment available.