When I was researching bamboo shrimp a while ago, I wanted one, they said that sifting through the sand is not a good sign and is a sign that there is not enough food for the shrimp. That is what studies have shown, I will follow the studies since it is multiple people that can indeed back it up. I am sorry, but Chocolatebetta is right, the bamboo shrimp is indeed starving, sifting isn't normal behavior.
I think the tank is fine and plenty enough room, but you really need to find a way to get the bamboo shrimp a way to get food from the filter/water column and have it sit in a perch.
Please take the proven advice and do something about it. Ignoring it won't solve anything right?
I want a bamboo shrimp so bad lol!