Sagging middle syndrome

No, I'm not talking about my weight and figure, although I sometimes feel that my middle could use some work.
I'm talking about the sagging middle of a novel.
It's a common dilemma. Many writers draft detailed outlines before commencing writing. Others write by the seat of their pants knowing their characters will see them through. My approach is a little of both.
I have found that if I spend the time developing my characters with an interesting back story so that their actions make sense and then come up with enough possible obstacles to throw at them at every turn, I can make myself at least the framework of an outline, enough to get me to feel confident when I sit down at the keyboard.
Of course, that doesn't mean I am not without fear. But at least I know I have something to hang my hat on should I run into a dry spell.
My current solo novel No Other Choice was suffering from sagging middle for the longest time and because of that I just couldn't get words on the page. I knew the story was there. I knew how I wanted it to end. I even had the big black moment all laid out. I just couldn't figure out what to put in the middle. What was going to keep my heroine from getting what she wanted and how was she going to arrive at the BBM [big black moment]. So the book laid fallow for a while which upset me. ThenI started brainstorming with what-if's. And it happened. The what-if's started flying off the page. If this happens, then that happens, which could cause that.
Aha! I had my middle.
Now, I can't wait to get to the computer every morning to put poor Charlotte through her paces. Writing is fun again.