Thank you for writing
After you update the value of the RadProgressBar in a different thread you have to execute a call to the following method:
and probably you will have to Refresh the form.
If this does not solve the problem, I would kindly ask you to send me a sample project which demonstrates how you are using the RadProgressBar.
I hope this will help. Should you need further assistance I would be glad to provide it.
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.