I find the middle challenging because it is essential to move the story forward, as you maintain the Ooomph you established in the opening; all the while laying the groundwork for the ending.
Also, if you are still working your first draft, you may have an idea of how the book will end, but you haven't written it yet. E.L. Doctrow said (and I'm paraphrasing) it's like driving in such thick fog that you can only see a few inches beyond your headlights.
Here’s where my final image, reflected in the final line, always saves me. It doesn’t matter whether I’m writing a flash piece of a novel. When I get stuck, I go to that final image. It pulls me forward.
Just keep plugging away, even if it's only for five minutes a day. You will have a lot of time to edit once you get this first draft down.
I agree the middle muddle is the worst! I’m less than a hundred pages into my fourth book, DEAD MERCHANDISE. So far, so good.
I did have trouble with the middle:not the actual writing but decided the plot was weak and ditched it, took at least a year to rewrite the whole middle section. Yet I still like being a pantser!