I would go back to the breeder's food of Purina Pro Plan. I would start with 30/20 Sport. Most Labs here do best on a food that is 30% protein, 20% fat, or close to it. If it worked at the breeder's, chances are it will work for you.
Orijen was way too rich for Luna. It is a very, very rich food. She's okay if we cut it with her regular food (Nutrisource/Fromm, depending on where she is in the rotation cycle), but if she gets any more than 50% her poops get pretty soft.
Is the skin itchy or is it just dandruff? A lot of Labs here have dandruff and do better with a fish oil supplement. Grizzly brand is usually recommended. I give both Grizzly pollock oil and coconut oil, but not at the same time. I would give the fish oil at a meal and not worry about the poops being looser unless it continues after giving it for 2+ weeks. Their bodies take time to get used to things, and that's okay.
I would also try reducing the food again by half a cup. Not gaining any weight shouldn't necessarily cause alarm by itself. If her body condition is good, she's sprightly and active, not gaining weight can be okay. Luna grew in spurts. She would be the same weight for weeks but keep growing and get lankier, and then put on a few pounds, etc.
You may also want to look into a probiotic. I use Probios powder just because the horse gets it too and I'd rather buy one 5-pound bucket and use it for them both than 2 different probiotics. A lot of other people here use Purina's FortiFlora with great success. With a probiotic, you typically want to keep it refrigerated, so keep that in mind.