Open up Tools/Options, go to Library & Folders and check "Run auto-import in the background" is ticked.
Next, click on Configure auto-import. Check the folders are added that you want Media Center to watch. Make sure they are accessible by browsing to them with Windows Explorer. Are you using UNC paths or mapped drive letters? If using UNC paths, make sure they are working or try changing it to mapped drive letters.
Select each folder and click Edit. Check the proper file types are selected and click OK.
Under Tasks, check the correct options for additional tasks are checked. If you have 'Ignore files previously removed from the library' ticked, untick it to make sure that its not that.
I've noticed sometimes it can take a while before songs are imported, but Im not sure why that is. Especially if analyze audio and build thumbnails are enalbed under tasks, it takes a while. I think that background process has a very low priority so as not to interfere or hog the system.