Unusable Paragraph Decrease Icon

I’m using Libre Office and compiling a 50,000 word document. Line spacing is an issue. But whenever I click on ‘Paragraph decrease’ to adjust my line spacings I find that the icon is grey (ghosted) and so of course it won’t let me operate it. Once in a while it DOES work (ie, it is ‘black’, therefore clickable), then for the most part it reverts to grey and therefore unusable.
Please, please explain - in simple terms please - what I can do to stop this.
Kevin Daly