I would assume that the UV
filter is killing the shrimps food but I can't tell you for certain.
They can dig at ornaments and rocks to get food, but generally that means that they aren't getting enough from the water. Not something they should be doing.
You could try to target feed them crushed flake food or I think they might have specific shrimp food, not sure. You would have to mix it with some tank water and shoot it into their fans while they're filter feeding.
I would double check on the type of shrimp too. The one in the picture looks like a Viper shrimp, I don't think there is much difference between them besides size though.
Freshwater Invertebrates for Aquariums: Singapore Flower Shrimp
Tropical Inverts for Freshwater Aquariums: Viper Shrimp