Are you actually getting communication with the ECU?
If not, on cars with the immobilizer function, an internal relay can fail. That relay is supposed to enable OBD-II communication with the ECU only after the immobilizer has verified that the key is valid. That relay connects the ECU's K line to the OBD-2 port.
The "quick fix" for this would be to connect the K and L lines on the main body control module wiring harness, making the ECU's data line constantly available. You can search for instructions on how to do that on this forum if you suspect this is the problem in your case.
Some OBD-II readers also have issues communicating with the ECU because of the comms chips they use; have you tried a different reader?