how to find fault?

The push button switch on these flymos is known weak point, and I appreciate you feel that is ok, but I'd double check this with a multimeter if possible and replace if any issues found, they are a very cheap part and easily available. (the supplier I found has sold over 11 thousand of these switches to date!)
Also, make sure the battery is holding a charge, once again a multimeter test should show around 40 volts across the positive and negative terminals. Might also be worth looking at the terminals where the battery docks onto the mower to see if you have any debris or a loose terminal.

Rupert
March 2020