how to coat truffles with chocolate
Working with chocolate is so tricky! 

Introducing even a tiny amount of water to melted chocolate will cause it to seize. The water could come from unexpected places: steam from a double boiler, condensation on the interior of a lid, the use of a wet utensil. Seized chocolate can be returned to a smooth, melty texture, but it will no longer be suitable for coating. It can be put to other uses, though. 

However, you may have overheated your chocolate, which will also cause it to seize - overheated chocolate can be saved, though. Blending overheated chocolate with solid chocolate, or pressing it through a sieve, may save it.