damaged goods

I bought 2 Camp master air beds on 22 Dec 2015. When we got home we pumped them up and saw that all was well and only recently during Easter holidays did we realise that one of them deflates very slowly overtime. We looked for any holes but could not find any. On returning the bed to the store I was told to leave it inflated overnight for their observation. Next day they also can't find any negligence or product abuse in my part but they simply can't exchange it. No reasons or explanations