It's a pretty common problem. Im not going to be too much help here except to say - ABSOLUTELY DO NOT LUBE ANYTHING.

Take the trough out, and try to 'push apart' the walls a bit, to make sure the balls have a little play. If it's too tight, that will keep the the ball that falls in from pushing the others back.

Sadly, this happened to many folks. (Hopefully this does not turn into a TFLE bash thread)

Anyway: One solution:

The code has an option for Megatron virtual lock. Feature adjustment 5.

This takes the fun out of locking 4 balls, but it would solve the issue.

Other Solutions:

Make the sure the game is pitched at 6.5 degrees.

Some people have widened the trough physically to make it looser and make the balls move more easily. Some have placed spacers under the trough to increase its pitch (remember you want the balls to roll to the back).

Good luck.

And I agree with markmon: DO NOT LUBRICATE ANYTHING

I only had this issue occur when the pf pitch was not perfect. If you increase pitch of your pf, you need to compensate with washers under the Megatron trough screws. If you have no idea what I'm talking about, make sure your pf pitch bubble is dead nuts center. If its still an issue try putting something under the rear legs to raise the pitch as a TEST and see if that resolves the issue. If it does, then Work with that and crank the back legs up for a permanent pitch. This game seems to play way better at a 6.9-7 deg pitch, but that's just my opinion.

I just got my TF and have my back legs up pretty high and haven't had any trough issues yet (knock on wood). I will take a picture of the bubble and measure the pitch when I get home. One thing about Sterns is there is no consistency from machine to machine, so what works for one person might not work for the next.

There is one other thing that could be holding the balls up and that is the switch itself. I had it happen to two of them. Something broke inside them and were very hard to push down holding the balls from rolling back.