There are two possibilities:
NW 7 introduced a new way to control the devices, called CDI (Common Device Interface) which is used by default. You may still use the old method. To do that, change the device parameter "CDI" from "SCSI commands" to "not used".
Especially if you used new tapes, this may de obvious. Whenever NW can not recover from consecutive write errors, he will expect bad media and marks it as full. This prevents further usage.