angelfish rejecting food

We got 3 koi angelfish recently. When I feed them they would fairly
leap out of the water for the flakes. However now one of them is
sometimes found hiding under the filter. When it's not hiding and I
drop the flakes in, it will come up to half an inch of the flakes,
then just look at it. Tried dried blood worms, it took just a nibble.
Tried thawed frozen worms, it swallowed one, spat it out, and repeated
that a couple of times, finally rejecting it. What could be the
problem and what can I try next? thanks...