Barbs eat shrimp, particularly small shrimp like cherries.
In reality, most fish will eat shrimp if it can fir in their mouth, but barbs in general tend to be pretty ferocious about it, and will sometimes just rip them apart.
I think your shrimp will either just be hiding (as you said your tank was heavily planted), or the odessa barbs have eaten them, possibly after they started to shed their skin (they often do this when there's a change of water conditions, and it leaves them very fragile for a while).