Command RaiseCanExecuteChanged-Problem

I have a problem with my Commands.
I have got a MainViewModel and a DialogViewModel. In the DialogViewModel I defined a Command, which gets executed when a dialog should be shown. I make this command as global command, by defining a static CompositeCommand and registering the DelegateCommand (I am using Prism, but it seems to me, that this is not really a Prism problem).

When I start the application I have a datagrid. Everytime the selectedObjects in this grid are changing I am calling the RaiseCanExecuteChanged-method. This is working properly.
Ok not perfectly, because the first object in the grid I select will not give me the expected behaviour! It will always give me a null value as parameter, although the RaiseCanExecuteChanged-method is called after I set my SelectedEntities-Property and my CommandParameter is bound to this property. When I click a second object everything is like expected:
The CanExecute-Method is called and I get the selected objects passed in as parameter.

I am not sure why this is happening. I am calling the RaiseCanExecuteChanged-method to ensure, that the databinding is correct at the time. Before I did that the CanExecute-Method was only called once at the beginning and never again. It seems to me, that the databinding at the point when I call RaiseCanExecuteChanged is not set up properly. But Why?
Here is an example of my binding:

<Button Content="Aufsplitten"

CommandParameter="{Binding RegionContext.Value.SelectedEntities}"

Command="{Binding Source={StaticResource DialogCommands}, Path=ShowSplitbuchungDialog}"

Style="{DynamicResource ToolBarSplitButtonItemStyle}" ContentTemplate="{DynamicResource ToolBarSplitButtonContentTemplate}" />

Has anybody a suggestion?