J.B.
Are you using special chocolate bark made for melting and dipping? It usually melts with a slightly different consistency and doesn't get runny. You can get big blocks of it everywhere from the grocery store to specialty shops.I've also had great success with melting Nestle's TollHouse chips and their white chocolate chips in a glass bowl in the microwave. Heat on high for a minute; stir well; heat 30 seconds and stir well again. See if you need any more heat -but go slowly.
If you are using that already and still having a problem, maybe you're getting it too hot or melted. It needs to be "dippable" but never runny or so liquid that you could quickly and easily pour it. If pouring, it should move more like "sludge" than water. You can experiment with adding very small amounts of powdered sugar to it as well to thicken it up -like you do to make icing -but be careful and add just a little bit at a time; stir well and see if it thickens a bit. Make sure the kitchen is cool and you're laying the dipped items out on wax paper on a cool surface -not close to the stove or anything. Good luck!