The montages displayed in the montage editor depend on the signals that are displayed in the selected figure, it may not represent all the montages that could be applied for this dataset.
The function that guesses which montages are available is the following:
https://github.com/brainstorm-tools/brainstorm3/blob/master/toolbox/gui/panel_montage.m#L1197
The part I suspect to be dysfunctional is the last test:
https://github.com/brainstorm-tools/brainstorm3/blob/master/toolbox/gui/panel_montage.m#L1256
If you are familiar with Matlab coding/debugging, you could check this out and suggest a better test.
In case the pre-selection is not satisfying, you can always display all the available montages by clicking on the button "ALL", just above the list of montages.
The problem is that if the montage you want to edit is not in this list, it would not show either in the list of available montages for the figure (in the Montages popup menu for the figure, or in the Record tab)