Lack of Shampooing:

“When you don’t shampoo — which would shed [hair] naturally as you wash — it means that the hair is just sitting on the scalp,” dermatologist Dr. Francesca Fusco told Shape. “You’ll see the accumulation when you do go to wash your hair.”
To eliminate any build-up on your scalp, Dr. Fusco suggests cutting back on the dry shampoo, as well as treating yourself to a weekly scalp exfoliation, which is simpler than it sounds. Simply mix an exfoliant like sea salt, or even sugar, right into your daily shampoo!

Scalp Condition:

Scalp conditions can sometimes hinder hair growth and trigger hair loss. Dermatologist Pamela Jakubowicz told Prevention that these conditions can include seborrheic dermatitis (which can be caused by a yeast overgrowth, hormonal changes, or excessive oil production, and results in greasy flakes), psoriasis (an autoimmune condition that produces thick white scales on the skin), or ringworm (which can manifest as scales or blisters on the scalp).
A dermatologist can determine the cause and treatment of these conditions and get your growth cycle back on track.

Read more: https://sapidsapiens.com/2017/04/14/reasons-why-your-hair-is-falling-out-ii/