I would have to agree on osmotic shock. Are you still using ph down? Before adding more shrimp I would do a 90-100% water change to rid the water of any unneeded products that could be hanging around. Definitely a must to test the water they come in. If it is off by a ph of 1 or more and/or tds level is off by 50 or more, spend a few hours acclimating them. Some may think this ridiculous, but I have acclimated RCS for 8 hours before. Ph of the water they came in was more than 1 lower and TDS 225 more than mine. One even got the "band of death" and then molted right out, the molt separating at that very spot. Don't get too discouraged, just try to learn what you can and move on. I think a whole lot of people kill some, if not all of the first shrimp they keep. Also after adding them to the tank, if you have to add water again, drip it into the tank. This will allow them to continue acclimating well and they will molt normally. Remember, patience is key! You can do this!