Concise answer
Laser targets suitable pigmented hairs during active growth, but not every follicle is in that phase at one visit. Fine, light or hormonally stimulated hair may respond less completely.
This is why treatment is described as long-term reduction rather than guaranteed permanent removal.
Session spacing, device choice, settings and changes in hair calibre influence what appears to grow back.
When evaluation may help
Reassessment is useful when there is little change after a properly spaced course or when new facial hair appears with hormonal symptoms.