Popcorn must have a small amount of moisture in it in order to pop. If your popcorn is old it may have dried out and then won't pop consistently. It will also dry out pretty quickly if it's not in an airtight container (ie: plastic or glass with a rubber seal). I had half a bag of popcorn that sat for a while (3-4 months) in my cubboard and when I used it again only half of the kernels popped (bags are questionable when it comes to being airtight). Long story short, it's not your popper that's the problem (in fact the first one was probably just fine) it's your popcorn.