I have been having problems with the Pro Mix and Fox Farms line lately also. I know exactly what you are talking about. It looks like burn, but it is actually nutrient lock out. Keep your pH higher. I make sure I seen lots of green when I use the dropper test. I expect my pH in a nutrient mix runs at 6.5-6.8. Fox Farms nutrients build up and slowly drop the soil pH and if you are already keeping it at 6, it is really easy to drop it below 6 with some nutrient build up. I recommend adding a watering between veg and flower and then bump up your nutrients. Also do a watering after the week 7 and 8 where you are loading with Grow Big and then bump the nutrients again. This will help prevent nutrient lock due to low pH levels, but also help you finish strong.