Call the factory. Maybe they didn't install all the required weights. There is a chart specifying required weights in my manual.
That's weird, cause now i'm looking through my manuals and on the break down for the machine it doesn't even show the front weight plate... But there is one on my mower... I'll keep looking though.. Also just saw that the 48" & 52" frames are the same part number, wonder if that means I could slide a 52" under my mower?