I begin using azelestine for post-nasal drip, I have a severe chronic sinusitis. It did work well in the first 2 days then became less effective. Why?

It's an antihistamin. Azelastine is a nasal antihistamine spray. It works very fast and starts providing relief in just 30 minutes. However, antihistamines do not help with all nasal symptoms. They help with the histamine triggered symptoms but there are many other mediators at play. Nasal steroid sprays are much more effective but take several days to work. Nasal saline rinses are also very effective for sinusitis.