In myt 15 gal. shrimp-only tank the Amanos are out all the time. I just got back from cleaning a friend's 100 gal. tank that has about 50 fish and 30 shrimp. The shrimp hang under the driftwood and don't come out.
I also think that water quality has to do with a reclusive behaviour. If the water is very clean - low ferts and organics - the shrimp seem to be very active.