Some foods like cherries, pineapple, and watermelon will "case harden" early on in the drying process if you start at too high a temperature. After that it is very difficult to get them properly dried for room temperature storage. Maybe that's the problem.

I use relatively low temperatures for drying most fruits and some vegetables and only crank it up for meats. I also just leave it go for an extra day, when I don't have time to unload. I found out the hard way that split cherries aren't going to get any better after two days if you start out to hot. Since I lowered my temperature, I can make raisin-like cherries.

I've had gooey batches of watermelon in the past, mostly because I cut it too thick. I now cut in thinner and get it dry enough to crack instead of bend. Still has some stickiness, but not enough to "mind meld" the pieces in storage.

I agree that it and similar dried food (tomato paste and other dried tomato stuff comes to mind) can start to rehydrate from the air when the package is opened and get nasty. Using more and smaller packages helps quite a bit when there is a lot to deal with and the package will be opened and closed many times.

There is hope...

-aj