DBSPI-0006 alerts when the NUMBER of Tablespaces with low space is greater than the threshold. The default threshold is .5, which results in an alert when there is one or more tablespaces with low space. The DBSP Install and Configuration Guide explains the calculations used to determine low tablespace. This documentation should explain what the requirement is trigger the alert.
I would think that changing the threshold to 0.0 would force an alert. You may try running the scheduled task that calls this monitor from the command line, preceeding the command with "sh -x" so you can see what is going on.
If you want to alert when individual tablespaces reach low space (identified by the number of available extents) you many want to look at DBSPI-0203. Again, the Configuration Guide explains this monitor.