If your chocolate is clumpy, there are a couple of things that could have happened. First, you may not have melted it properly, to begin with. The chocolate needs to be smooth before you start or they aren't going to turn outright. The other thing is that the chocolate could have seized up. This happens from heating too much or if liquid gets into the chocolate. If this happens, it is best to throw it out and start over. Cooks Illustrated offers a last ditch option to reverse the reaction that you can try when you are out of chocolate and this is your last hope.