Check engine light does not turn on

I bought a used car. Unfortunately, I could not remember whether the check engine light worked when I was inspecting the car to buy. I bought an ELM327 scanner and it was not communicating with ECU. Then I bought another scanner tool (KW820) which also responds "link error" and unable to communicate with ECU. When I was searching for this in internet someone asked to check the check engine light, then I found it does not work. Car service center mechanic still can scan my car with the laptop connected to the OBD port via a cable.

1. Why my check engine light does not turn on?
2. Why can I not scan with those scanners?