Unexpected column break in sub-summary

[Alan H]

I searched and couldn't find an answer for what seems like a mighty simple problem....

I have a simple list organized by one sub summary field. There are two tables: taxa and families. Every taxon is a member of a family. There is an integer that I sort families by and taxa by. I then print with a sub-summary in the layout, followed by fields for each taxon. (See attached). It is printed in two columns.

When I print (go to Preview mode), sometimes there is a large space at the bottom of one column and a sub-summary break to the next column. Sometimes there isn't. I can see no rhyme or reason why. I have "Allow part to break across page boundaries", but that doesn't seem to affect this problem.

I'm using Filemaker Pro Advanced 11.0v4 for Macintosh.

Any thoughts?