Quote
DaveA
My printer is down right now so I can't test it but I think I remember that a G29 would do nothing unless preceded by a G28 immediately before. This is with Marlin BugFix release from about a month ago.
Yes... You should do a G28 before a G29. And the UBL system will automatically do the G28 for you if you invoke a G29 without first doing a G28.