Thirding the 'don't get it right, get it written'. Get words out, editing can happen later :)

*attempts to help*

I find it helps not just to write down notes there and then, but to write something down that paints that idea for you, an exchange of dialogue or turn of phrase that's what made you want to write it and will (hopefully!) bring the imagery/sounds back into your head when you come back to it.

Maybe don't sit down staring at a blank page determind to write a story? Maybe just type out your scraps and then a few more scraps, a funny line, a joke, a quote, something that catches your interest like rope (rope seems to be on everyone's minds at the moment, no idea why *grins*) and see where it takes you?

Or start off by writing something else. Sometimes I did a few fills for comment_fic prompts that got me going and then dove into something bigger :)