I had a similar problem with a mystery novel back in 2013 or so. I found that my projected plot was far too complex. When I cut out one subplot, I lost a couple of characters (and good riddance), and it all smoothed out.Started reading my own novel again. It kind of got derailed at the 16th chapter (roughly 35k words in this sparsely written first draft), as I painted myself into a corner somewhat, and I couldn't find my way out of it. It's been sat untouched for about a year. Maybe more.
I'm reading it through again with a view to untangling it, and allow it to flow freely again. I have the ideas now to get it back heading in the right direction, but need to sow the seeds in the earlier chapters, so that they can take root, and grow and weave through the later ones. I hope to have the first draft completed, and have made a decent start on the sequel, by the end of the year.