Making Carmel Slices (Australian dessert)?

I have made Carmel Slices a few times now, and everytime I make it, the Carmel Filling never is firm, it always ends up goopy and runny. When I was in Australia I bought one at a bakery/coffee bar and the carmel filling was firm and was not thin and runny. I've followed the directions from the recipe the same everytime, am I doing something wrong or is there another ingredient I should use? I tried refridgerating them after they are made and doesn't help either. Any thoughts? Thanks!