Late reply, obviously, but for anybody with this issue, perhaps it is because you have not yet learned the recipe? Hit the Experiment button when you open the mortar and pestle, and add one of each of the ingredients, then hit Combine. If you successfully combine it into the poison, you will have also learned the recipe. Close the mortar and pestle and reopen, search for the recipe, and go from there. I hope that was the solution to the problem.