Why does my hair stop growing right above and under my scalp?

It's unusual but not unheard of for hair to stop growing just above or under the scalp, especially if you're seeing a mix of normal hair growth with shorter, broken hairs. Since you're taking Finasteride and your doctor noticed this pattern, there are a few possibilities to consider:
Shedding Phase (Telogen Effluvium): Finasteride can cause temporary shedding as part of the hair cycle. This shedding phase typically occurs in the first few months of treatment, but if hairs are breaking close to the scalp, it could be a sign of telogen effluvium, where hair enters the shedding phase more abruptly and appears shorter or broken.
Hair Cycle Disruption: The fact that some hairs seem to stop growing just above or below the scalp could suggest a disruption in the growth cycle (anagen phase), leading to shorter hairs that don’t fully mature. This can happen due to factors like stress, hormonal changes, or an underlying scalp condition.
Trauma or Hair Breakage: Hair breakage can sometimes mimic this pattern. The broken hairs might appear as if they’ve stopped growing, but in reality, they’re snapping due to weak strands. This can be due to excessive friction, harsh styling products, or scalp issues like dryness.
Finasteride Adjustment Period: While Finasteride helps prevent DHT-related hair loss, it can take 6-12 months for visible results. The thinning you’re seeing may be part of the initial adjustment period where hair cycles are resetting, causing temporary thinning before regrowth kicks in.
To promote regrowth, Minoxidil can be a useful addition. It works by directly stimulating hair follicles, increasing blood flow, and prolonging the anagen (growth) phase. Many patients see improved density and faster regrowth when combining Minoxidil with Finasteride.
If you’re still concerned about these broken hairs, consider a scalp examination for conditions like miniaturization (common in androgenetic alopecia) or telogen effluvium. Keeping track of your progress and adjusting treatments accordingly can help restore balance to your hair growth cycle.

A good scalp examination by a dedicated hair can probably help solve this. If this is not possible, then a biopsy will be helpful. trichotillomania, alopecia areata and traction alopecia are the top causes in this case-but of course there are other possible causes too. See an expert.