Why do my shrimp continue to die?

Ghost shrimp aren't a good indicator of tank health... A lot of the time, in the store they are close to death. Keep in mind they are sold as fish food.

It shouldn't be hard to tell on the ammonia... 0 is a very bright yellow, if there's any hint at ALL of green, it's got ammonia in it. If you added 10 minnows a week ago, it probably went through a mini cycle.

Your BPS on co2 means nothing... You need to get a drop checker with 4dkh solution and regulate your co2 based on that, BPS is a pointless measurement.

I would let the tank cycle and then grab another 20 cherries.

How big is your tank and how long has it been set up. Shrimps are very sensitive to water conditions. When i set up a shrimp tank i dont put any shrimps in untill its a month old and usually water change with aged water and not declorinated. Then i drip aclimate them for a hour till its 90% tank water before release