Yep, be sure it's not right out of the fridge as others note. If you add milk or butter, I would be very cautious -- I suspect that would make the melts "seize" and turn un-fixably grainy. Check on the Wilton web site; there may be a "frequently asked questions" section that could help you!
If you want to keep making molded candies for anyone other than young kids (I make them for us to sell at my daughter's ballet school performances, and to give as gifts), I recommend you switch to Merckens brand candy melts--you may have to order it online unless you can locate a candy and cake-making supply shop in your area. MUCH better quality candy melts with better texture and flavor, Ifind. Yes, Merckens costs more but is tastier! But Wilton is OK for kids' stuff if they aren't picky....A good candy/cake making store also will have a ton more molds than the craft stores that sell Wilton melts, plus flavorings, bags for candy...really fun to explore! I have done candies that were Egyptian pyramids and gods, tools, angels, Easter Eggs, and a lot of dance-themed ones too, thanks to molds!