Prolonged cough with mucus. With time.

I am suffering from cough from six months along with phelgm/mucus in throat. Sometimes I suffer from runny nose, sore throat and heaviness in chest also. I also feel sleepy during the day. At times there is dried blood in my left nostril. My blood reports, chest and nose X-Rays are all normal. I have consulted various physicians, ENT and chest specialist, tried various medications and cough syrups but it only works till the time I am taking them after that the problem starts again. Moreover I don't suffer from fever, inflammation of neck , blood in cough or weight loss. I am covid negative. Some doctors say there can be allergy from some particles in air, some say it can be sinuses but there is no concrete evidence. Please let me know what can be the problem or cause of this problem. I am really concerned as the condition has worsened with time.