I sometimes feels that I can't take in enough air. What can be the problem?

Sometimes is the key. People who always feel that they can't take in air likely have a restrictive lung disease like pulmonary fibrosis, muscular weakness due to muscular dystrophy, or copd. People who sometimes feel that they can't get enough air may have asthma, anxiety or periodic spasm of the vocal cords known as vocal cord dysfunction. See an allergist, pulmonologist or otolaryngologist for assistance.
Many things. This could be many things. Asthma, infection, muscular pain, heart failure, pulmonary hypertension, inflammation due to chemicals or certain dusts, smoking, bronchial restriction, tightening of the trachea, blood clots. It all depends on your medical history and the circumstances around your symptoms. Please see a doctor for a thorough work up.