A problem like this one could occur if you are using a custom / external skin and you did not include the main RadFormDecorator CSS file. If this is your scenario, please add the main CSS to the page and everything will work as expected.
If this is not the case however, we will need a sample project that reproduces the problem. Please prepare such project and send it to us in a support ticket - we will check it right away.
the Telerik team