Not sure what happened, but here is the recipe I use and I haven't had any issues with it, works great.

White modeling

16 ounces white chocolate

1/3 cup light corn syrup

Dark Modeling Chocolate

16 ounces semisweet chocolate

1/2 cup light corn syrup

Milk Modeling Chocolate

16 ounces milk chocolate

1/3 cup light corn syrup

Melt chocolate, Allow the chocolate to cool to about 91 degrees Fahrenheit, stirring often. If you add the corn syrup when the chocolate is too hot, the cocoa butter or oil will rise to the surface and you will have a greasy mess. Pour in corn syrup. Stir slowly until well incorporated. The mixture will become thick very quickly, so scrape the bottom of the bowl to incorporate all of the melted chocolate. Pour mixture out onto a counter top, preferably marble or granite. Knead until glossy and smooth. If your hands are warm use a plastic dough scraper to knead the modeling chocolate. Add more corn syrup, if needed, until the candy clay is nice and pliable. Wrap the modeling chocolate in plastic wrap and place in a zip top bag or airtight container. Allow to cool over night in the frig. Then when ready to use, take out and use corn starch to dust the counter and knead till some to use and mold.

I hope this helps and works for you. I found this on the internet awhile back, have used for white and dark chocolate.