Stu,
This is not the current problem since you're using a ML1200 or when using the serial port but since we were discussing Ethernet connected SLCs yesterday now is a good time to bring it up. For SLC 5/05s and Micrologix 1100 and 1400 over ethernet another reason online edits sometimes don't work is an incompatible configuration in RSLinx. In RSLinx go to communications -> configure CIP options and set both the Max Fast Messaging connections pre PLC and Max Slow Messaging connections per PLC to 1 and then click Apply. If you haven't already done so then its something you should do as you proceed with networking those PLCs.
I have ran across that also on slc 5/05 but bot all of them so I am thinking it is firmware specific. Very strange things happen with this issue when doing online edits.
I got these settings from rockwell but the tech could not explain to me why this was happening or why those changes had to be made on some processors.
Do you have any more detailed info as t why this happens and why those settings help?