Some measures can only be partially selected?

Hi, I have an arrangement I'm making and, for some reason, some measures can't be selected entirely. Most can, but then some leave a gap at the front of the measure, where a clef may be displayed, if there were a clef change. (See attached image.)

I have changed the clefs back and forth as I've continued editing, so it's possible some of these measures previously had a clef in them, and was later removed, but I'm not sure.

I've tried highlighting the measures and telling it to remove system breaks (Format -> "Add/Remove System Breaks..."), to no avail.

Any idea if these will cause problems at any point, or how to remove the gaps so they operate like all the other measures in the score?