Can singulair (montelukast) help with post nasal drip and thick/plugged mucus (no nasal discharge)? been taking it for perennial rhinitis for 2 weeks and no help.

No it won't: Singulair (montelukast) helps reduce inflammation in the airways, upper or lower, helps as an adjuvant therapy on the long term, not meant to treat acute or chronic bacterial/viral infection of the respiratory tract. See your doctor for evaluation, possible chronic rhinosinusitis. You may want to see an Allergist/Immunologist if you have access to one or an ENT specialist, best wishes