Phlegm is something many people experience occasionally, particularly during a cold, allergy flare-up, or respiratory infection. But what if the phlegm keeps coming back even after you seem to have recovered? Persistent mucus production can have several causes. Sometimes it is related to post-nasal drip or allergies. In other cases, recurring phlegm may be associated with asthma, chronic bronchitis, COPD, respiratory infections, smoking, environmental pollution, acid reflux,