Forum:Column width problem in monobook

Hello, I have a problem with my main page: if you use the monobook skin the right column is not 300px width, although I set it like that. Do you know why? The templates in it are set with "100%" width, so they will have the same width as the column, anyway this is a problem that occurs only in monobook. Do you know why?

Oh, I tried using the "mainpage-right(left)column" tags, but it's no use. Also this is the code:
 * }