I tried the water based stuff years ago and I think it is junk. I have never had the real stuff fail, but I have had a similar occurance like what you had where it didn't stick right. I don't know the exact reason, maybe you are right about the humidity, but you don't want to have such variables that will cause something to not work. If it doesn't work sometimes then it is junk. Maybe you didn't stir it well enough and the stuff on the bottom didn't stick as well. What I would do is create better ventilation. If you purchase a decent window fan and put it in the window, and completely seal around it, then set it to "exhaust" so it is blowing out, then you will be able to comfortably use the good stuff. Or, you can wait for a nice day and do it outside in the parking lot. My never-fail trick is to put two coats of regular contact cement on both the substrate and the formica and then put them together (waiting, of course, for each coat to tack up correctly with the dull sheen before either re-coating or assembly).