In SharePoint 2010, you might run into a situation where the "Reanalyze now" is disable and there seem to be no way to fix it.

In my case I fix it with the following steps:
. Go Central Administration then select Job Definitions
. Then look for the job that has a title "... On Demand ..." and has a status of "disable" and click on it and delete it

You should be able to run the "Reanalyze now" afterward.