This issue occurs for me constantly in Mac Pages. When I want to update a style, such as changing line-spacing or fonts, the "Update" button will not appear next to the selected style in the format section. Therefore I cannot make global changes to the style.
Sometimes I am able to "force" the button to appear by changing line spacing to a different number then click Update, then change it back, then click Update again. But this does not always cause the Update button to appear either. The appearances are totally random.