If you restart the agent on the active node this will initialize a new discovery. After this you can check the Operations Manager log which should have some warnings and errors in to help you find why SQL isn't being discovered.
It is also worth checking that your Action Account or SQL RunAs accounts (if you have specified them) have the relevant permissions. You can get more information about that in the SQL MP guide.
Also, have you upgraded to SP1 yet and do you have the latest MP version?
Hope that helps, post back if you still have problems.
David Allen
http://www.inframon.com
Microsoft MVP - System Center Operations Manager 2007
WMUG Community Leader http://www.wmug.co.uk/blogs/aquilaweb