Fudge Crumbly?

I make fudge all the time. It is the recipe on the Hershey can with a little alteration. My question is, at times I make it sometimes it turns out perfectly, but other times when I cut it, it is crumbly. I make it on pretty days, but it still comes out crumbly at times. Other times it stays together nicely when cut. Weather is not the factor, but I don't know what is. I always measure carefully. Any answers out there?

By GR