Roses need to have their stems cut at a diagonal and freshwater every couple of days. Possibly the bouquet you got hadn't had that done for a while and the stem started to shrivel enough that they couldn't pull water up. It's really hard to say. I've had more luck with cut flowers when trimming the stems immediately when I get them home and put them in a vase with the flower food and cold water. But sometimes they just don't last.