I followed a recipe I found in a (last year's) magazine, for chocolate ganache. 300ml water and 125g caster sugar brought to the boil and poured over 250g chopped dark chocolate. The recipe said whisk until smooth. I whisked for AGES and it just never thickened!! Eventually I left it in the fridge for a couple of hours and eventually it was thick enough to spread and allow to drip down the sides of my roulade - still softer than I expected. What did I do wrong??? Thank you.
That may produce some sort of chocolate sauce or confection, but ganache is by definition made from cream and chocolate.
Basic ganache is equal parts chocolate and cream by weight. Given that ganache is typically made with whipping cream (30% milkfat), the water being added is about 70% of the weight of the chocolate.
Your product contains more water than chocolate, not less.
So there are likely two problems:
I suspect the too much liquid issue predominates.
Next time, make the ganache from the traditional 1:1 ratio of cream and chocolate. You can find many good sets of instructions such as these from Alton Brown at the Food Network.