retroredscottie, I have had the same problem with "super glues" (cyanoacrylate glue). I have read that they go bad with humidity. So now I keep mine in a jar with a couple of packets of desiccant.
I have a real love hate relationship with Pavelka's glue. I love the fact that I can re-bake pieces that contain glue. However, even keeping it in a jar with desiccant, doesn't prevent it from getting thick and gummy a lot faster than regular super glues.
2 part epoxy works. I did have an experience with it not setting up. Maybe I didn't stir it enough. I used the kind that extrudes equal parts, so incorrect proportions wasn't the problem.. I got so annoyed that I went back to using E6000 for my pinbacks. I don't have a problem with it not sticking to the clay. I do buy the little tubes of it, and make sure that it is fresh. As it ages it gets thick. It may not work as well then. I usually score the pinback and clay a little with my Exacto knife and then clean the surfaces with alcohol. Then I apply it according to the directions - thin coats of both pieces, wait about a minute and stick them together. I get a thin coat by putting the glue on the pinback, placing it on the brooch, then pull it off. This leaves a thin coating on both surfaces. Then after about a minute I put them back together.