Coating chocolate (aka candy melts: Guittard A'peels, Merckens, Wilton Candy Melts) will not melt to a proper "thin" consistency if they are too old.
On the Wilton site, an experienced decorator recommended: if you need to thin down thick candies, add 2 tsp shortening to 400g (14.1 ounces) candy melts
The BEST product to use to thin candy melts is Paramount Crystals. Paramount Crystals are available online or at specialty cake stores.
Personally, I think Wilton Candy Melts are terrible!!! If that's what you're using, that's probably the reason the candy is too thick. I took a candy class and we went thru about 12 lbs. of Guittard A'peels. They melt beautifully and should be the consistency of pancake batter when you lift your spatula.