Why Are Symptoms Of Cough And Common Cold Appearing Once A Month In A Child With Allergic Asthma?

Hi doctors, my baby is suffering from allergic asthama. He used budecort 100 2 puff twice a day for 11 months and 1 puff twice a day for one month. He shows symptom of dry cough once a month and uses asthalin 2 puffs 4 times for a week. Why the symptom of cough, common cold is appearing once a month? What is the best medication to get rid of this?