Concise answer
Transplanted follicles are selected from relatively resistant donor areas, but no hair is immune to ageing, illness, scarring disease or poor donor selection. Existing non-transplanted hair can also continue thinning.
A result may look less dense later because surrounding native hair has progressed even when many grafts remain.
Long-term planning therefore considers donor stability, future loss and maintenance before the first procedure.
When evaluation may help
Review is useful for new patchy loss, scalp symptoms, donor thinning or a gradual change that cannot be explained by comparable photographs.