Bald spot 18 months after FUE hair transplant?

A doctor would need to examine you and give you an assessment based on where you had received the transplant and where you have the bald spot.

You need to get an opinion of someone that can look at this in person. Although pictures might have helped us here.

A photo of the bald spot would be helpful, as well as a photo of the area before the procedure.  Was the area filled in with the hair growth from the procedure, and then you lost the hair, or did it never fill in? 

Photos of the bald spot area before & after transplant would be helpful. However, it's a certainty that you need to have an in-office evaluation either by your surgeon or another qualified doctor.

It is impossible to determine without actually seeing the area. I would recommend that you do an in person consultation to get a full assessment. It may possibly be an area of Alopecia areata. This can be treated and is only temporary.It may also just be further loss of native hairs that will continue without some preventative treatments

If a bald spot the size of a coin appeared, a doctor should look at it and see if it is a case of alopecia aerate which would do that