What are some reasons that fudge fails to harden?

Not beating the fudge mixture long enough or not cooking the mixture at the right temperature may prevent the fudge from hardening or setting. Another reason that fudge fails to harden is that the fudge did not cook for a sufficient amount of time.

Fudge fails to harden when the sugar, butter and milk ingredients in the mixture do not get hot enough during cooking, possibly due to use of a broken candy thermometer or the lack of using a candy thermometer. Fudge sets correctly when stirred thoroughly, so after removing the fudge from the heat, the cook should beat the mixture until it thickens.