Your ball seems to hesitate before it starts to roll after the gate activates - I wonder if the wood is dented a little at the switch slot? That might be delaying it moving and allowing the gate to catch it.

THanks heaps for that!

OK...I've got that plunger so I'll put it back on again.  The wire gate is off, mine is slightly different and it gets stuck.  I would have to make one by hand I suspect.  I will check the wood around the switch to see...

I'll report back shortly.