Writing a critical review of a journal article - I found starting on a new document, and starting from scratch gives better insight. Also structure each paragraph/section with focussed questions And just answer those questions.
I find stepping away from papers enormously beneficial. Sometimes I get a bit stuck or run out of steam and park it for a week. Other papers I've had on the back burner for months and when I return to them it's as if I'm reading something written by someone else! Writing is a creative process, sometimes your subconscious needs space to process your ideas - separately from articulating them on paper. Obviously this isn't an approach that is helpful when you have a firm deadline, or are working with others, but it works for me. Depending on who your coauthors are, and your relationship with them, in my field you could definitely send an unfinished draft with a note saying that you'd got as far as you could and would now like some fresh ideas as to how to develop it further. If anything, this usually generates too many ways in which the paper can be advanced, not fewer! Good luck.