Chatting to a shrimp breeder here in Singapore earlier today, he told me that failed molts can be due to too high water hardness. I always thought it was too little minerals, but he said that too high GH can cause failed molts too. His recommended parameters for CRS are 3GH, 0KH and TDS of 80 - 120 max. I measured the water that my new CRS came home in and it was actually 3 GH, 0 KH and a TDS of only 66! He recommended 5GH for his neos. Not saying that these parameters are in any way right for your shrimp, but just throwing the idea out there given your 8/8 water that too high minerals in the water could also be a possible cause....maybe.
The advice @Zoidburg
gave above regarding diet is worth heeding. After reading his comments on a previous thread I checked the ingredients on my various shrimp food any found that even the algae wafers have so little algae in them, and are mostly fish meal. Actually pretty hard to find a high % algae based food! Hoping this is going to be the final piece of the puzzle needed to get my CRS tank thriving, fingers crossed.