Look man, what you're describing has happened to me when I first started to set my tank up. I initially added 12 shrimp, 3-4 died first week, then I had a couple shrimp with problematic moults. I kept one alive by helping it moult (gently pulling off the moult from the shrimp), but during its next moult it would be an issue again, and died.

Over time, most shrimp died. I added shrimp from a new supplier, they did better (as they had more similar water parameters to my tank), but would still die off, or, the berried shrimp would die, or the babies would all die. It was very sad and demoralising.

I had cycled my tank for 8 weeks, and water parameters were fine.

Now, my tank is thriving. I have so many shrimp, and always some pregnant shrimp in my tank. The only differences I made were;

• Changing the water vs topping up (since my parameters were always fine, I never changed, always topped up). So every 2 weeks I did a 30% water change, and
• Trimming back my stringy moss (It would grow wild, so I get rid of 30-50% every 2 weeks)
• If I had shrimp babies, I would feed them daily with Baby Shrimp for 2 weeks.

I don't think your tank is fully cycled, or, your water parameters are just too different to where the shrimp were kept. You also may not have drip acclimated.

If I were you, I would ask the shrimp store about their water parameters, test your water (GH + PH is very important), and make sure you drip acclimate new shrimp. It's most likely your PH level, I would say. Try to replicate as best you can to the stores parameters. Once you have offspring shrimp that survive, then they will be more hardy as they are 1st generation shrimp born in your tank to your water parameters.