The reason for this behavior is that when the appointments are placed in read-only slots, actually they are in read-only state and their read-only style (greyed-out overlay) is applied. You should customize the AppointmentStyleSelector and in both Horizontal and Vertical appointment templates edit the ReadOnlyVisual element.
I have attached a sample project showing the approach using Implicit Styles
. Please download it and give it a try.
Hope this helps.
All the best,
the Telerik team