We finally had to take our pup off Fromm due to the same issue. We had over four weeks of either super soft stool or straight up diarrhea. We also tried probiotics, adding rice, a tablespoon of pumpking now and then, nothing worked to firm it up. We even got meds from the vet that didn't help. We went on vacation for a week and during that time, gave him plain chicken and rice for several days and his stool improved considerably so we decided it was the Fromm. We are currently changing him over to Orijen and it's too early to tell if that is going to be better (but his farts right now will burn your nosehairs OMG). We too debated long and hard over switching him again but after talking to a lot of dog people and seeing his stool improve on the chicken immediately, we decided we needed to change. If the Orijen does the same thing, I don't know what we'll switch him to. Maybe kibbles n bits since it seems the really expensive food doesn't work (kidding of course).
I've heard at least a few people say that the Fromm was too rich for their dog but you'll have to decide if that's your issue as well.
Good luck, we know how you feel!