I've had it happen to me too. You probably have a small crack in the workbowl. There are small tits that are on the "small plastic piece" that snap into place on the workbowl, but when it cracks, it won't hold.
You need to use "gorilla glue" to hold it in place. Be sure to clamp well after inserting because the spring puts pressure on the "small plastic piece". LET SIT 24 hours after clamping.
catsnjags, August 2009