This is a link to a site I'd saved just yesterday, as I'm about to embark on my first try of making modelling chocolate.
The following paragraph is an excerpt from the above linked article, which may explain the problem.
UK chocolate is VERY different from that available in the USA. UK chocolate is sometimes referred to as "vegolate" by the EU as it contains such a low quantity of the actual ingredient that makes chocolate -chocolate. Chocolate from the USA does not suffer from this problem. This means 100g of american chocolate contains a much higher quantity of chocolat than the UK product.
I'm really nervous about trying it now!
It worked fine for me with milk chocolate - tesco cake covering chocolate. No problems there at all. I'm sure you'll do fine. And thanks for the chocolate facts - I learn so many new things here