New backup option not working when legacy worked fine.

We have been using cPanel servers for a couple years now and have always used what is now the Legacy Backup to perform backups of our clients on a weekly basis. After the current backup options were released we updated our settings to use the new option. We set up 2 backup locations: one for a local backup server and one for Amazon S3. We currently have Amazon disabled but both locations validate with no issues. However, we are continually receiving trouble out of the local backup. It will transfer fine for a while, then suddenly quit. We'll receive no email errors but we will notice not all of the clients were transferred, and that they are still under the /backup directory on the server. Or, we will get an email error saying that it failed to authenticate with the local FTP server and that the backup was disabled. But when we check in the latter instance, again, some of the domains will have transferred correctly, but the rest will still be on the server. The backup server (FTP server) is always running. Other servers back up to it fine still, and we've never had this issue on the legacy backup. Is there something I should be checking, or possible settings I've overlooked, that may be contributing to this issue? If I manually FTP the files through the terminal it works just fine as well. Thanks, James