Group and Outline function in Excel

What’s going on here? In the same workbook in Excel, the Data/Group and Outline function works fine on one tab, but not on another (the little plus and minus don’t show up to hide the columns). I’ve tried hitting “Clear Outline” and “Show/Hide Detail,” but no luck. The settings seem to be the same as well. Any ideas?

Is the sheet perhaps protected?

I don’t think so, when I go to Tools/Protection, it doesn’t look like it is protected. I’m wondering if the file is somehow corrupted. It used to work a couple weeks ago on all of the sheets. Now it works for all of the sheets except for one.