Plots out of Control?

Or is it rather that this particular plot is being more finely tuned?
I’m currently writing my third novel in my Peace Child trilogy. This is possibly the novel where for me the plot has been the least clear. I do know roughly how I want the story to go and how it should end, but it is becoming increasingly difficult to know exactly what the next step is.
I’m not sure exactly why this is happening though I suspect a few reasons. One must be that this time I am not working towards the end of just one story but towards the end of an over-arcing story that takes place through three novels. Secondly, my time and brain-space are more fragmented these days because of my job as a university lecturer. This isn’t a complaint about the latter – I love the work and it complements my work as a writer very well. However, it does dominate at times and I then require even more self-discipline than normal to keep on task. I hope I’m right about a third reason: I am a more competent and more experienced writer and know even more about plotting than I did when I wrote my last novel. There is a constant growth as we learn the craft, unless we become jaded, and I don’t actually think I have.
Yet I feel increasingly insecure about this plot. I find myself constantly poking in details and am continually struck by new ideas. Could it be, though, that I am being more open to a creative process? I know anyway that I often do my best writing when I’m feeling less secure about it. That also, I guess, is because I’m being more open to suggestion.