Are you able to post what is in your content file for us to have a look at. Also do you edit the file inside mercury where you have the option to click "check syntax"?
Can you place a rule at the very top of the file that you know will trigger
if header "from" matches "*@*" weight 99
and see if it does, if it does then place it at the very bottom and see if it still triggers.
I recall that mine stopped working once even though mercury reported it not having any syntax errors and it turned out ( I can't remember exactly) but it only stopped working after a certain point in the file. I found the problem by simply moving a rule I knew would trigger in different spots in the file.
Hope that makes sense.