I start at 20 - 22 brix, buffer pH with calcium and keep it at about 5, use plenty of nutrients, keep it 80f or warmer, and never get a stuck fermentation. I do about 500 gallons of wash a week for the last 4 years. So, I would look at it from the beginning: what is your water chemistry? What exactly is your feedstock? How are you adjusting, monitoring, and maintaining pH? What are you using for nutrients? Fermentation temp? Normal finished gravity?