Either it's your cake that isn't cool enough, or it's your ganache. For good measure, pop your cake back in the fridge (or the freezer) for a few more minutes and let your ganache cool down some more before trying to add the drip. You can also try making your drips smaller. This recipe yields a bit extra ganache, so experiment with your drips on something besides your cake before beginning (sometimes I test my drips on an upside-down bowl before moving over to my cake).