Check the following:

• Have there been changes or updates in Full admin? Any changes to the CSS/templates can cause these issues. See issues with template and skins.
• Has there been a change to the styling?
• Have there been any feature changes? These can affect the overall UI.
• Does the issue happen on all browsers? Check browser compatibility. What browsers are the users using?

If none of these are the cause of the issue or you get an error message, contact Support  for assistance.