My best guess would be that you didn't quite get the tongue of the drivers seat under the tab quite right. I'd remove the seat and try again. Make sure it snaps in good at the front where the locking tab goes into the slot.
I don't have a locking tab.... I guess it was left off. I see the holes where it should go, but it is not there. I didn't notice with the passenger seat on because it held it snug. I thought the seat was a little bouncy but just thought that was normal. :rant-rave: :rant-rave: I guess I'll call my dealer but i'm sure the part will be on backorder