I used to work at a store that carried these. I hated that my management ordered them. Anyhow, I had a fondness for shrimp even then and I tried to take extra good care of them. Sometimes we'd get them in as you described-- keeping their fans clamped and not eating by filtering OR picking. I would make sure they were in a tank by themselves with varied hardscape in case they were stressed. I wouldn't sell them when they were like this, and sadly they always perished within a week. There was no information about them online back then so I never knew if there was more I could have done. I do hope someone here chimes in that has more experience with these guys. I'd love to know if you can help them once they are at this point.
I believe these guys are still harvested from the wild so it might be very hard to pinpoint a cause.