Eyebrow follicles are slow growing anyway so it takes time for them to recover from any disruption in growth. Skin grafts are often have small areas of graft loss from the build up of the fluid right after surgery. Mostly it is a question of patience. Of course if the cause of the thinning is still present then the thin growth may persist. If the burn injury did not damage the eyebrow hair root, the eyebrows will regrow. If the burn injury destroyed the hair root, eyebrows will not grow back.
 
     Eyebrow Regrowth tips

Blood vessels to increase the flow of blood and nourish the follicles.

Effect the hormones, which can stimulate hair growth. in the case of hair growth will not be limited to brows, if the product work.

Continued use of Rogaine twice a day for at least four months is usually needed before you notice hair regrowth. Eyebrow follicles are slow growing anyway so it takes time for them to recover from any disruption in growth. Rogaine should only be used if you are experiencing hereditary hair loss.