You may see fewer backups on your server or remote backup destination than there are accounts being backed up.
You may see an error like this in your backup logs.
Available disk space (4 percent) is too low. Backups will not run. at /usr/local/cpanel/bin/backup line 1251.
This indicates that the disk that the backup system is using is running low on space during backups. As such, further backups can not be created.
Increase the amount of space available for the backup system.