Have you tried (with the whole page selected) Utilities -> Music Spacing - Apply Note Spacing... and finishing with Update Layout (same menu)

Thank you for the suggestion, Michel, and I tried what you wrote, but it does not work in the least. Might there be a glitch in the program, or something I did that set this measure to look like this? (I have tried to rewrite it, and the same thing happens.)

What's happening in the fifth staff down in the measure in question. The image is too small for me to see clearly, but something seems off there.

The only way to figure it out is to examine the file (or an excerpt). You can post in on a server, like Dropbox, Google Drive, etc... and put the link here.

The note duration was correct, but apparently when I examined the 6/8 measure's components (beat duration), it was reading it with eight beats instead of six. I manually corrected the measure, and now it looks much better. Whether it will play correctly is another matter, but thank you, J Adrian for calling that to my attention re the clarinets. 

I doubt that was the problem. Try Staff tool, clear staff styles for score and parts for that measure stack.