Make sure your tahini isn't too dry, as it will make this difficult to mix. If it is a bit on the dry side, add a bit of oil, maybe zap it in the microwave for a few seconds, and give it a good stir.
On a similar note, the recipe mentions removing the tahini mixture from the heat immediately when everything is melted and blended. This is important, otherwise the tahini might start to crust up and you'll have a hard time mixing it in with the oats and quinoa.