This is my second day of reapplication of cosmelan mask. Why is it not peeling off?

Cosmelan is a great product and I generally recommend two treatment 3 weeks apart as the company recommends for best results.  Cosmelan re-application 4 months later should have had similar results.  I would suggest you return to your treating physician for evaluation.

Generally
peeling happens when there is a buildup of excess skin on the face; if you had
a Cosmelan Mask 4 months ago it is possible there is no buildup to peel off.
The Cosmelan product will continue to improve your pigmentation as it
penetrates into your skin and inhibits your melanin production.

In my office if there is left over pigmentation after a
Cosmelan treatement I usually recommend the continued use of Cosmelan 2 at
bedtime and an anti-oxidant vitamin C/brightening creams in the morning. At that point my patient can also do a light
laser called Clear and Brilliant Permea to accelerate the improvement of the
left over pigmentation. Best improvement is seen after a series of 6 sessions
done once every 3 weeks with the use of vitamin C anti-oxidant and SPF 50 in
the morning and a lightening cream at night, my patients are very happy with
the improvement of their pigmentation on this regimen. Consult with a Board
Certified Dermatologist to see if these options will work for you.