The job starts all right but it copies the backups form Saturday the week before???
Please describe you source job settings: start time, retention. The thing is that when GFS to tape starts it waits for one of three things to happen:
1. If the source job creates a full backup, the GFS job copies it
2. If the source job creates an incremental backup, the GFS job synthesizes a virtual full backup
3. If no backup appears in 24 hours, the GFS job looks for the most recent restore point available. If it is a full backup, the GFS job copies it. If it is an increment, the GFS job uses it to synthesize a virtual full backup
So in your case the following should occur - your GFS starts on Monday and sees no new restore points. Then it waits for a while, your source job generates another increment, GFS takes it and synthesizes new full backup dated "Monday" (case 2). What you described looks like an unexpected behaviour so please contact our support team and post your case ID here.