My Schipperke had a similar problem following a visit from another dog (relative from another state). The vet eventually said it was a combination of Low Thyroid combined with a flea problem. We used her monthly flea med. (remember to always wait 3 days before bathing and 3 days after before applying.) Many people bathe their pet then apply flea med. Instead of waiting 3 days after bathing before applying. This took care of our problem. I also applied some mild hydrocortisone cream to ease what seemed to be itching and to moisturize her skin.
There are many reasons of losing hair. You did not mention other symptoms that you noticed. Are there patches on its body? Does it scratch too much? It is one of the reasons. Other reason can be due to endocrine glands disease. Sometimes hair just become brittle and breaks off. May be there are fleas present in its hair that you haven't found yet. Give it bath regularly. If your pet has itching and hair loss then it is Feline Atopy. May be it is just a seasonal hair loss if there is no other symptom of any disease. But still if hair loss continues then it is better to consult a vet to clarify the situation.