Generally if you want to enable such kind of validation (aka "asynchronous validation"), perhaps you need to set ValidatesOnDataErrors = InViewMode, this way the user will be able to see the errors and edit another item.
Indeed a sample project will be the best option in this case, but any information that could bring more light to the problem is welcome.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items