Are you able to post a sanitised version of the imccfgbakdm log file?
Also, are these devices using SNMP read/write?
If devices support SNMP read/write (and IMC has an SNMP device adapter for that device), then it uses an snmpset to get the device to upload its config via FTP to the IMC server.
If it doesn't support SNMP (either no r/w, or no SNMP adapter), then it will connect via Telnet/SSH, and enter the required CLI commands to tell the device to back up via FTP.
IMC will then watch the output of that session, and when it thinks the backup has finished, it will exit. IMC then copies the backup file from the tmp directory (aka the ftp root). The key bit is "when it thinks the backup has finished" - it guesses this by watching the session output, and looks for something like "File transfer successful."
The problem is that the message they look for doesn't always match what your FTP server sends. So sometimes the backup file has been FTP'd, but IMC thinks it never completed properly. Since IMC thinks it didn't finish, it just leaves the backup file in the tmp directory.
I'm not sure if that is exactly what's going on, but that's what I'd be starting with. The imcfgbakdm log file should be able to tell you more.