There are many reasons for flat nails, including iron deficiency anemia, lichen planus, and onychomycosis. 1. Iron deficiency anemia: due to insufficient or excessive loss of iron intake, symptoms such as fatigue, dizziness, shortness of breath, pallor, lack of luster of the nails, and flattening of the nails can occur. 2. Lichen planus: this is a kind of inflammatory and itchy skin disease that can involve the skin, mucous membrane and hair follicles. Typical lesions are purplish-red flat papules, which can also lead to flat nails. 3. Onychomycosis: It is a skin disease caused by fungal invasion of the nail plate or under the nail. The main manifestations are nail thickening, separation, atrophy, flattening, buckling and other forms. There may be other reasons why the nail cover is flat, if this situation continues unrelieved, it is recommended to go to the hospital in a timely manner, after a clear diagnosis for targeted treatment.