Why long transaction cannot rollback?????

There are two IBM RS/6000. The model are 370 and 380. They are all installed Informix On-line 5.0. The dbspaces are same. Only Share-Memory differ in Max User#. There are 12 logical logs. Every log is 1,024Kb. In 380, the long transaction can rollback. But 370 cannot. Now LTXHWM =50,LTXEHWM=60. When a log tansaction happened, the 12 logical logs are 100% used. Informix is holded on there, cannot work anymore. The message log showed " logical logs are full, Need to Backup." I used Auto-Backup, but cannot backup, and still cannot work. Check the user flag C-R-----, the mean is wait for a checkpoint and process a rollback. Why cannot rollback totally??? When I changed the LTXHWM=80, LTXEHWM=90, the same thing was happened. When I changed LTXHWM=10, LTXEHWM=30, only 2 logs full, the long transaction was happened, and rollback totally. But only 2 logs is too short!!! What happened??? What should I do to let it work normally??? Madeline Wu from Taiwan....