Guppies are severely, severely inbred. They can up and die randomly, and it is very frusterating. I have found that Delta guppies are the hardiest by far, as well as sunrise guppies. Try buying from a different lfs, and experiment around as to which store has the strongest guppies.
good luck, i gave up on guppies for a while, then started back up, with the right fish from the right store, and I haven't lost one in a while now. I have 15 gorgeous guppies and counting, as some are pregnant. Once you can find a nice crop of guppies, they are extremely hardy.