Hi Skeat,
When they ship the plastic tube runner things for the table they are rolled up in a box & I think they crack or get damaged there. I did have to change out some of the plastic runners earlier on & Gracie Co. sent new ones real quick. It was hard to get runners that didn't pop up out of the track a little so I decided to use some 100% silicone caulking & fill up the inside of the runner tubes before setting them down in the track. It was a little messy & involved some clean up but I know that 100% silicone sets up like rubber & I thought it would make them better than original in strength to be more solid in order for a heavy machine to roll on them. I did this about 1 yr ago & haven't had a problem again.