I am going to side with Rivermud on this one. Not that my opinion counts for much. I am surprised your water is so hard considering the driftwood and plants in your tank. Perhaps you should throw in a couple of more smaller pieces with some more plants. This would not only increase the softness but increase the hiding spots that would alleviate some potential stress they may be feeling.
I would double check everything. It was just today that I found one of my 5 gallon tanks had its heater unplugged, I found this was a big reason why my fish was so stressed =/
Is there a filter (a fine sponge of some sort) over the intake into your HOB filter? My ottos used to get sucked into my ac50 in my 20 gallon, this definitely stressed the hell out of em until I added a filter for them and my RCS.
Do you have substrate? If so, what kind? I am not sure if shrimp, like certain fish, prefer the presence of substrate.
I would list off several other things that I'd check but they are all basic that you probably have done: photo period (making sure the inverts can sleep at night), background, position of tank, foot traffic, I might also ask other people in the household if they are doing anything with the tank (Hey, ya never know!), and... things like that.
Good luck, and let me know what the problem ends up being
Oh, and maybe you need to introduce some genetic variation!