Small patch of hair won't grow back after I shaved it. What can I do?

Without seeing you scalp, I can't really tell what the cause is. There are quite a few possible diagnoses based on the scenario you have presented.  Shaving does not lead to permanent hair loss in most cases unless there is some underlying process. If it has not grown back for a few years, there is a good chance it is not regrowable. But see a hair specialist for options, including non surgical and surgical.