Hey,
this seems pretty strange. I had to monitor also some mount points a few weeks ago and did the same. I had no problems with alerting.
A couple of what might be stupid questions: Have yoiu waited long enough? Are there any related events (Errors, Warnings) on those agents?
What ifg you increase the MB Free Disks Space to over 1TB? Not that the calcs are done differently.....
Regards,
Stoyan