I think much of it has to do with how the flowers have been treated in the shop/ florists. For example, I've noticed that some roses, usually bought from the same super market, last a lot longer than others. On some occasions they gradually open up and last over a week. At other times they last 3 days and the heads, still in bud droop.
I treat them the same, snipping the ends, putting the flower food in and usually placing them in the same room.
This would under normal circumstances be headline news
