Why isn't my face wash working?

Ok so a year ago I went to my doctor and she prescribed me Clindamycin Phosphate and recommended a certain face wash to use with the prescribed stuff. Well it worked great up until 3 or 4 months ago. I stopped using it for a month or two (I've done this before and when I started using it again it worked just as it did before) and started using it again in the beginning of October and it hasn't been working like it used to. Why is it all of the sudden not working anymore? I'm 16 now if that helps