Cannot unlock 'greyed-out' icon in Libre Office

I have copied and pasted 100s of emails onto a Libre Office word document. My ‘Decrease Paragraph’ button is greyed-out for about 50% of the time when I try to decrease line spacing. Often it will ‘turn black’ and be operative, then ‘grey out’ on the same email.
Please help!

If you have a very large document you should adjust the memory setting as a start. OPTIONS > MEMORY. The default settings are for standard short documents which make up the majority of use. This is especially true if you include images or a large number of text pages. There are a number of answers dealing with memory.