If you make a question and someone asks you to try something, after you do it where should you put the results? For example you could always edit your original question to put the info there or if the request to try something was made in a comment to the question you could reply with a comment or if the request was in an answer you could reply with a comment to the answer. What is the most clear way of proceeding? Which approach notifies the most people that more information is available on the question?
Update the question.
Comments are, by their nature, impermanent. They're supposed to be used to ask for clarification from the author and to otherwise coordinate improving the question. (The same goes for answers.) Once they've completed that task comments may (and should) be deleted.
If, of course, someone posts a comment that gives you the solution, you should ask them to make it an answer so you can accept it. (If they don't in some reasonable amount of time, go ahead and post the answer yourself. It'd be nice if you could give credit.)