Why some OBD tools don't pick up any error codes?

Each time my car got issues, i run to different shops to scan my vehicle. They all use different diagnostics tools but only 1 out of 4 will actually find an issue within the vehicle. Some shops use more than one diagnostics tool. I always ask not to delete error codes, so that if they cant pick up any issues,... show more Each time my car got issues, i run to different shops to scan my vehicle. They all use different diagnostics tools but only 1 out of 4 will actually find an issue within the vehicle. Some shops use more than one diagnostics tool. I always ask not to delete error codes, so that if they cant pick up any issues, maybe the next shop will.

Why is that ?

Car is a Hyundai Tucson 2009, not a Tesla or Bugatti...
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