Maybe the bearings aren't quite as "free" as they should be...
Soak them in AeroKroil. Lube them with 3 in 1. Spin them with a drill.
heh.
Also check the springs underneath the bearings. (I dunno if yours has those, but in a Wico there are small springs). THese springs help keep the ball as close to the opening as possible, without it really rubbing against it. If those springs are compacted, then your gap will be a bit bigger than it should be, allowing the ball to jump up.