There are many things happening when I'm generating ideas and working out a picture (I'm guessing others have a similar experience). I'm sorting out thoughts in my head, maybe writing down key words or phrases to help the idea along, and trying to make the pencil or other media make marks.
If the mark making tool I'm using is difficult to use, getting the idea out on paper feels more difficult. For example, if I'm trying to write and the pen I'm using gets stuck or drags, so do my thoughts. On the other hand, if the pen I'm using glides across the page, I experience more flow of thoughts.
I'm not saying there's a perfect tool out there that will allow the ideas to flow freely and effortlessly and every picture will be AMAZING. What I'm suggesting is this, if getting ideas out seems overly frustrating, maybe experiment with different media, different tools. I try this out sometimes (I'd like to experiment more).
Other sticking points are that my skills aren't up to where my tastes and idea expectations are set, or that I haven't used a certain media enough to be comfortable with it. I'm thinking both of these will take practice, not being afraid of my mistakes, and a plentiful amount of play, experimentation, and doodling to get familiar with the media. I hope to remind myself of this next time I start a project.
Then there's what a tool does for me. Right now I know I think best when creating thumbnail sketches with pencil rather than on the computer. Using a soft pencil lets me skate across the page and makes getting ideas out a little easier. For getting myself in a more open, playful mindset I'm liking kid's crayons which require some effort to make marks because there's some serious drag on the paper and when layering crayon, but I'm willing to trade that physical awkwardness for the mental playground.