Retin-A Not Working. WHY?

I used Retin-A last summer and it worked great. Face went from drab to dazzling in a matter of 2 wks. I experienced little peeling... I then took a break from it in the winter. When I started up on Retin-A again recently in mid-April; however, I found that it isn't working the way it did last summer. No mild peeling from the Retin-a and my skin isn't glowing the way it did from the product. I'm wondering why this is... Please kindly advise. Thanks.

No doctor answers yet