We have made the standard type of these for years, using a recipe similar to this one 3-Minute No-Bake Cookies. A problem we have sometimes is that the sugar will not set (I think). The cookie should be slightly crumbly and have a granular(?) texture. However sometimes the cookies will not set properly. The cookies remain gooey and will not harden even after a day or two. As far as I can tell, the same recipe is followed each time. Possible variables might be boiling temperature (by eyeball, not using a thermometer), the timing of removing the heat and adding the oats, or the humidity (we are in a high (70%) RH area, so some ingredients might be affected). Also we use Nestle Nesquik instead of cocoa. Also margarine instead of butter. I would appreciate any insight as to why this may be happening. ADDITIONAL INFO. There is one additional difference from the recipe. We mix the boiled mixture with the oats in a separate bowl, rather than adding the oats to the boiled mixture in that pot. Possibly the cooling time is an issue. UPDATE After further experimentation, the best result was obtained by using a Dark Chocolate cocoa powder (which I presume has a higher cacao content, although the products I used did not state any percentage). Even the result of other milk chocolate cocoa powders was not as good. UPDATE 12/19/22 Finally got around to making another batch. This time they came out almost perfect. Only two changes from the previous method.  Heat sugar/milk combo really slowly. Maybe 4 (where 5 is Medium), and leave there until the margarine cube on the top is melted. Then raise the temp. just enough to keep it boiling. (cherylee50 mentioned this, thks). In my recipe, then boil for 6 min.  Use a full 8 oz. marshmallow creme per the recipe. I was using only 7 oz. as that is the jar size these days.   
This is what I found to work every time. I always use real butter, not margarine, and real cocoa, and I only add just enough milk to make a thick "mud". Just follow your regular recipe as usual , but only put enough milk in to be able to stir your mixture. Bring to a slow boil, stirring constantly to ensure your sugar melts evenly so it won’t be grainy. Bring to a full rolling boil for one minute and you should have perfect no-bake every time. 