Thanks for writing.
I have tried to reproduce the issue by creating a Windows Phone 7.5 project and adding the Telerik tools to the Toolbox, upgrading it to a Windows Phone 8 project and re-adding the Telerik tools to the Toolbox without experiencing any issues.
The exact steps are:
1. Create a Windows Phone 7.5 project and add the Telerik controls from the Primitives assembly to the Toolbox.
2. Upgrade the project to Windows Phone 8
3. Remove the old Telerik references and reset the Toolbox.
4. Add new items to the toolbox from the Primitives assembly by following the same routine as before.
Can you please share details about your approach? Having the exact steps that you're taking will be helpful in determining the reason for the behavior.
All the best,
the Telerik team
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