No-bake cookies won't harden

[Suns88]

My friend and I have been trying to figure out why our no-bake cookies won't harden and have had no luck. There are only six ingredients; milk, sugar, butter, oatmeal, peanut butter, and cocoa. Sometimes they do harden and other times they won't.
Does anyone have any ideas based on the ingredients that would cause the cookies to stay so soft?
I'm curious if regular oatmeal versus quick oats would cause that?

 

[PA Baker]

Moving to the desserts/cookies forum...

 

[letscook]

I had a problem with fudge and peanut brittle, come to find out I brought the store brand sugar and it is a finer sugar. You need to have good ole Dominiino's sugar. Also the sugar might be a beet sugar not a cane sugar. When I bake I always use dominino's Haven't ever had a problem since.