Threonine - critical amino acid for membrane formation. Not listed on guaranteed nutritional analysis (or at least, not required to be listed). Look for feeds high in Fish meal, corn gluten meal, soy meal, or soybeans.
As a temporary thing, go get a big thing (5 oz) of nutritional yeast from Wallyworld, cost you about $5. Mix with 5# of feed. Feed till done. Evaluate for changes in height and stability of the yolk. If you have a good grocery, or organic is very important to you, look for Anthony's Brewer's Yeast Powder - its a little cheaper per ounce, but a larger bag.
If that works, you've identified your problem, change feeds to a higher protein option and be sure to provide plenty of free choice oyster shell. Changing to higher protein feed will be cheaper than trying to uniformly add enough yeast each batch.