Group Function Works Sporadically

[Mashuganah]

Sometimes when I select a group of rows and use the "Group" function it works
correctly and groups them with an "+" toggle to collapse or expand the group.
Other times the command indents the group but does not create a toggle.
Repeated attempts to use the Group command on a given group of rows does not
work after it fails, even when other sections of the spreadsheet are
correctly grouped.

How do I make the group command work consistently?

 

[Mashuganah]

I found that I can get the "Group" command to work by adding two blank rows
after the group on which the "Group" command would previously not work. Talk
about arbitrary.