Speaking generally, the RadListBox has a VirtualizingStackPanel
as its ItemsPanel, which means that only the items which are currently in the viewport will be generated.
May I ask you to check whether you have placed the control in a element, which measures it with infinity, as this will disable the UI Virtualization? Such elements are StackPanel, ScrollViewer or Grid's Row/Column with Height/Width set to Auto. If that is the case, you can either place the RadListBox in an element that does not measure it with infinity or explicitly set the Height property. Please, give this suggestion a try and let me know how it goes.
If the above suggestion does not help, may I ask you to share some sample code demonstrating how the RadListBox is defined, so that I can check the scenario on my end?
I am looking forward to your reply.