The dash is for any user's session where AirWave never learned the SSID. Usually, this is because we only got the user info from a trap, and by the time we polled the controller, the user was not there. When we poll a controller, we get a much more complete set of data compared to what we get from a trap.
If you look at your list of currently connected users, do you see users who have a dash for the SSID and a value of "Trap" in the SNMP Source column? If you do, then this is almost certainly the cause.