From my experience. Do not wash the apples..just brush them with a clean paper towel. If they have any moisture on them, caramel will not stick. Also, I melt in microwave with a tiny bit of water. Just enough to stir the caramel. The best ones I ever did were caramel, then dark chocolate followed by crushed peanuts dipped-Yum!
Hope this helps..you can also try and put them in the fridge to harden faster.