Update:-

Replaced U20 (thanks to Gemini2544 for the chip) but unfortunately the fault persists.

To recap : CPU is reading Right trough switch intermittently closed when balls are in the trough . This is preventing game play.
All other switches in the row (and associated column) work correctly.

I rechecked the resistance of the row/column wire between the trough switch and the first switch in the row and found it a bit high (more than 1 ohm) compared to the other trough switches , so bypassed that wire by soldering a wire between the NO of the right trough switch and NO of the left trough switch (they are in the same row)
I previously replaced the switch and the diode (yes , new one is right way around) .

The switch tests fine with a multimeter .

What else can I check .. any ideas anyone as I have run out of ideas.