Beowabbit

Sounds like yeast failure to me. I know I found using my own yeast, that I kept refridgerated seemed to have made that problem stop. I think the yeast that comes with breads often has been too heated and the like in the shipping process, or perhaps extends its shelf life. Assuming you're doing the thing of keeping the yeast on the top and what not, but you seem the type to follow instructions. :> So try using your own yeast instead (available at any supermarket).