Cannot use any keyboard shortcut to go to next/previous section

Let's say I insert a section using ctrl+shift+R. It shows up as # -------------------------------------------------------------------------.

I make several of these multiple pages apart. These headings correctly show up in the "Jump To..." drop-down menu below my code (alt+shift+J).

When I use the default keyboard shortcut to go to the next/previous section -- ctrl+PageDown/PageUp -- what happens is identical to what happens when I press PageDown/PageUp without ctrl: it shows me the next "page" (e.g., if I was looking at lines 134-156, I now see 156-177).

I have checked to make sure that nothing else is assigned to the same shortcuts.

I have tried assigning unique shortcuts like ctrl+alt+shift+U, and again, what happens is the same as when I press U: it just types the letter U.

I have no problems using ctrl, alt, or shift in any other combination as shortcuts (e.g., ctrl+shift+C works as comment out, ctrl+Enter works as execute current line, alt+shift+J shows me section headers that correspond to the sections I added using ctrl+shift+R).

Any ideas about what's happening here?