SQL Server Maintenance Plans Not Working

I have Windows 2000 / SQL Server 2000 with a separate maintenance plan for the system DBs, and two other DBs on the server. Only one of the maintenance plans appears to be working properly. All appear to be configured the same except for differentstarting times. What would cause one plan to work completely, backing up DB and trans logs, another plan to only back up the DB and the third plan to do nothing?

When several DBs exist on one server are they usually combined into one maintenanceplan or are should they have separate maintenance plans?