You have to make sure the job properties are the same for each job to be nested.
Things like material size etc, what trips me up a lot is the quality setting for gradients.
Even if one or more of the sub jobs does not have any gradients the setting still has to match.
Any one thing can cause it to not nest.
Easy way to do this is set up and save a basic preset and apply it to all the sub jobs you want to nest. Once they are nested you can override the parameters at the top to the settings you want.
I am at version 8.6 but this should still apply to v10.
Flexi seems to be really sloppily put together but it runs my printer and guess that is as good as it is going to get.
wayne k
guam usa