my Baratza Sette 270 grinder has problems. the screen timer counts up but does not grind or will do one short burst during the grind time. if i 'bang' the top/back of the machine during the grind time it will temporarily grind. I have cleaned out the burrs etc so i do not know what the problem is. Any help or suggestions please?

Since physical abuse gets it going, however briefly, the most likely problem is the gearbox is shot. It's an easy and relatively inexpensive fix and a known problem. Over time, the plastic gears wear and skip a cog. Hitting the machine might get it to start, but it again skips when it hits a bad spot.
You can test for this by pulling the motor/gearbox assy, and removing the motor. This will leave the gearbox free to play with and you should soon enough figure out if the gears have a lot of back-lash.

Another possibility is the power board has a loose wire on it. This is another known issue. If you go to the Baratza site, you can find advanced electrical troubleshooting there. A common electrical issue is with a coil that breaks due to vibration. The fix is to unwind a turn and resolder, then put some silicone seal to stop it from vibrating. Those are the only two things I can think of.

Forgive me for being rude, but baratza is known for amazing service and the first thing I would do if I had a question would be to call them. In fact the FAQ in the baratza site actually describes this exact issue.

https://baratza.com/sette/

Check the limit switches for the interlocks first.