Config fetch initially works the same for all device types. That is, the protocol order used is set globably per application. For the devices where you do not have username/password configured, RME must be fetching the config using an alternate protocol like SNMP/TFTP. For those that are failing, either the alternate protocol is not supported by those devices, or there is another credential issue.
The protocol order is set under RME > Admin > Config Mgmt > Transport Settings.