scan tools, sensor readings?

(04 Cavalier)
I have a Gore Proscan OBD2 interface module, which lets you display
various sensor readings on a PC. Some of these (eg, rpm, TPS, ...) work
fine. But, ...

(1) Some sensors, which I know the PCM reads (eg, fuel tank pressure),
seem not available for display.

(2) Some sensors that the software shows as available (eg, fuel level)
seem not to show any readings.

It could be me, though I'd be the last to know that.

I would appreciate any insight on the availability of sensor data on
various scan tools. The vendor web sites seem to list the obvious ones,
and conclude with "And more ...". Is it just a matter of how much you
pay for the scan tool? ISTM, once you have the OBMD interface and user
screens done, the effort to support additional sensors would be minimal.
Would the PCM would hold back data the OBD? ...

Thanks