Hey Sparky, good to see you. Yes, when using straight Pro Mix you often have to feed by around day 10 - 12. Even though Pro Mix does have starter nutes, they don't seem to be sufficient for auto seedlings. I often see the same thing happen to mine and usually give them a light 1/4 strength feeding on their 2nd or 3rd watering. I also keep a spray bottle handy with a mix of 1 part household ammonia to 10 parts water. If I see their color getting a bit light I'll give them a spritz with that solution, usually just once is all that's needed. I'd also suggest you drop the pH of your feeds a bit lower, to around 6.2. Pro Mix is usually right around 6.9 - 7 right out of the bag. Feeding at the lower pH will bring the pH of the soil down to a good level by the time they start flowering.