Why would Cosmelan make my melasma worse?

I have melasma on my upper lip. For the past several years I have done cosmelan with success every winter. However this year when I went into my dermatologists for the second touch up application the before/after photos showed since the cosmelan my melasma had gotten worse. I'm so frustrated. I did notice on the cosmelan box that it was almost expired. I did the mask in November, the cosmelan was apparently compoundd in august and the expiration date was a week away. Could this be the reason?