hi Chris,
right click on the subreport that's not working and choose Edit Subreport. now go to File > Page Setup and ensure that these page and printer settings match the working one.
also, go back to the Design mode of the main report and ensure that both subreports are the exact same width in design mode.
you are correct that there is no setting for the number of columns so the method that you are using to get the desired number of columns (i.e. setting the column width) is the best one.
if the above doesn't work, then go back to the subreport and then the Section Expert for the Details section > Layout tab, and ensure that the Printing Directions and Gaps match.
if all of those don't fix the issue, can you attach a screen shot of the preview mode so we can see exactly what's happening.
-jamie