Often when I plan, I get shiny new ideas to make my storyline better. Ideas such as "What if X met Y?" I love 'What if ...' and usually, I'm able to determine the circumstances in which the 'What if' would happen.
That's when I get blocked.
Not all the time, but it happens often enough that I've developped a way to get me out of it. It's not complicated. I just write the scene.
Most of the time I put next to no description, and very little actions. I'm a dialogue person, and since they are usually what moves my story forward, they are what I focus on. The goal is to get a rough scene out, to write in circle until I find out where this is supposed to go. My mind does it for most of the novel, but when I get stuck, I need to write things down to get somewhere.
In a way, this is a lot like what I do with my first drafts. I'm testing the grounds. I just let the creative juice out, knowing this will never make it into the novel. It's a great way to work out my storyline.
Hopefully it will get me past my current knot. I'm almost done replanning!