@RonjaBiernat
My experience of these things and the answers, if it isn't a bug, is that it will be as a result of one of 4 things:
Too much water;
Too little water;
Too many nutrients;
Too few nutrients.
I find these issues deeply annoying! Had something similar last year which I think had to be as a result of something in some manure I'd got - but I could never work it out. Hope someone has some ideas.