When I first got my tiger pistol shrimp, it was a larger specimen that was sitting at the LFS for months. The owner did not even knew it existed but sold it to me anyways.
Long story short, I got it home and it instantly paired with the YWG. It either starved or something but it died about a week later. My suspicion was that my sandbad was so clean that it had no microfana to hunt or feed off of. It could also be that the shrimp was so acclimated to the tank that the shift it tank and parameters stressed it out.
Anyways if you got a smaller baby one, it probably is either hiding under the rock or digging some type of hideout.
The hermits on the other hand, probably ate each other. My hermits are always either killing something or themselves.