Can't adjust note spacing within measure

In some measures, I can get notes much closer to each other than in others. It seems to have something to do with where a line break occurs in a staff, but it's maddening when I have two identical measures with radically different spacing and can't get them to match up.

In the .png file below, you can see that measures 37 and 41 have identical content, but I can't get the notes in measure 37 to move any closer together. I select the note and take the Leading space well into the negative range but nothing moves. This happens often in many different contexts, but the rhyme and reason of when a note will and won't move is a complete mystery to me. Please help!