Sometimes when I melt chocolate it clumps together. Is there a reason for this? Should I use a different type of chocolate?
Actually, your problem doesn't have anything to do with the kind of chocolate you use. What chocolate hates is moisture and extreme heat. Even the slightest bit of moisture will cause that clumping. I've seen it happen many times when cooks try to melt a huge bowl of chocolate bits over a water bath. The problem starts when they don't turn the flame down after the water comes to a boil. Between the moisture of the steam, the boiling water splashing and the high heat, that chocolate seizes up harder than your great-aunt's fruitcake.
Now let's say you do everything right (or so you claim), with no moisture or extreme heat, and your chocolate still conspires against you. Well, don't throw your wooden spoon across the kitchen in despair, because all is not lost.