ntbackup keeps only one copy

hello
i use ntbackup to make incremental backups on a w2k3 sbs server (not the backup provided by sbs server management) but though i have set up the copies to be append (i need to have 5 incremental backups on monday, thusday, thursday, wednsday and friday) the system keeps only the last incremental backup, deleting the previous. Do i miss something?
thnks in advance
John