Why would an iron-on patch fall right off?

I've had a pile of "mending" for quite some time now. I'm not much of a sewer, so I was SO proud of myself when I got out the iron-on patches and patched some pairs of pants. Not only did I accomplish something different, but I also resurrected a couple of my favorite pairs of pants. Well, that excitement was short-lived when the patches just fell right off. I followed the instructions on them to a T. Now what should I do?? Did I get faulty patches, or cut them too small? Should I try iron-on patches again, or is there a better solution?

Thanks,
Dana