Skin boost [client]

I was in an ARAM match a few days ago and played amumu. When my teammate bought a skin boost the chat said I got surprise party amumu but I could not use the skin and my client says that I do not own it. This happened a while back with miss fortune, I got water loo but it never showed either. As I already had a miss fortune skin I didn't really care and therefore did not report it. However I would really like to have an amumu skin.

Thanks.