The workshops probably put it in a corner of the flat lever car park and turned it on
So if it is a loose wire or chaffed wire there is no movement to make them contact the frame
In some cases they may not ever have the blades running so no deck vibrations either .
In these situations I put a pair of orange in line spark testers on the plugs and take the hood off then go mow so I can watch them
If they continue to glow then flash after the engine starts to slow down then I know I am chasing a fuel problem
If they stop flashing before the engine stops turning then I know I have an electrical problem.
Now you can do that and get back to us with the results.
If with the testers in place the problem never reoccurs then the coils are weak, or you have the wrong spark plugs fitted