How to indent or tab List 1 paragraph style

Running LibreOffice5 (5.0.3.2) on Windows 8.1.

Have created a document that contains a List 1 paragraph style. It works, but for some reason I cannot use the “increase indent” button or TAB key. Nothing happens. Actually, it moves to the next item in the paragraph style list (although my cursor is in the document). The strange thing is that this ocuurs in just one document. When I open a new document and use List from the paragraph style, I can increase and decrease the indent as musch as I want. This makes me think the behavior must have something to do with the document. I’ve been researching this all day now. All in vain, it drives me nuts.

Any hints, tips or suggestions anyone?