Ideally you need a moderate to strong current for bamboo shrimps. They are not the most mobile of shrimp. Up until recently I had one and he always stayed where ever the current was strongest. With internal filters he would sit on the filter or powerhead and stick his fans directly into the output. If your tank has too weak of a current I could see them trying to scavenge which isn't proper for them. You might want to look into a powerhead for the tank. If you think you have enough current then give them a few more days to settle in. They often find a place they like and will often sit and filter there all day or until the current changes they will go hunt out a new place. If you think they need food in the water either crumble up and algae wafer or what I do is stick a wafer to the intake on a powerhead.
I'm not sure what went wrong with mine as I lost him just yesterday after having him for a long time. He had been moved around lately and I put him in a soil based tank I had setup a few days earlier. I noticed he had molted which isn't abnormal. But 12 hours later I found him on his side in bad shape. Something went wrong but IDK what. When he died he was still soft, which I don't think is normal since it was quiet a while since his molt. I want to say it was something in the tank, but the cherry shrimp seem fine and water tests don't show much.