We had this happen with our fudge recipe - the one we used was from the 60's. It was with Choc. chips, marshmallow cream, etc. then one year it was all wrong - it was dry and crumbling and so bad we had to throw it out. We tried it several times, buying different brands. Finally decided we had to change recipes - no idea what was wrong.
I do melt choc. chips with butterscotch chips to make our Chinese Noodle candy and have been doing that since the 60's and so for they are still coming out fine.