When editing a toolbar button in customize mode, checking "show label" does not cause the label to show. Why not?
Select the toolbar in the Settings / Customize / Toolbars list, then look on the right. It probably has Label State: Off, which overrides individual buttons.
That solved the problem. That there are two well-removed-from-one-another places that control the label display, that made it hard to discover. I know now though. Thanks.