Without knowing exactly how you melted it, I’m not sure if you did anything wrong. Usually when it gets grainy it’s because the sugar became too hot and scorched. I always melt mine in the microwave, but you can certainly do it on the stovetop (or even oven) too, over low heat. If your microwave heats unevenly, this might be better.
I begin microwaving the entire bar (in the tray it comes packaged in) for 1 minute. (Then I let it just sit in microwave for about a minute), then stir it very well (even flip over the square pieces) and microwave for an additional 15 seconds – that does the trick! Sometimes with a lower watt microwave it may need another 15 seconds.
If you live in a humid climate or there is a lot of moisture in the air, this can affect the coating – then simply stir in a tablespoon (or 2) of Crisco or even vegetable oil to thin it out.

Please reply so we can get this issue resolved! I would hate to see anyone having difficulties.
Thanks for your comment!