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.
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.
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.