Quoth Nikolai:
Ya, but you see, that dosent work well if you cant in-vision it. if you cant see what ur going to draw form in your head, thair is no way that you will get it on paper. (I know from experience)
That may actually be the problem. You're trying to come up with a concrete image before you draw it (at least, that's how I'm interpreting what you're saying).
Think of coming up with something to draw like running a car engine. If you want the engine to run smoothly, you need to warm it up first and get the oil lubricating the engine, but racing down the highway immediately upon starting the engine forces it to work harder and possibly even make it seize up because the oil isn't circulated enough.
Your brain is similar. It needs to be warmed up, perhaps by doodling in your sketchbook until an image bursts into your mind. That's what they mean by "getting the creative juices flowing". Those creative juices are the oil for your artistic brain.
If you're stuck on an image, you may be trying to force it through a part of your artistic brain that is, for some reason, lacking lubrication of creative juices. Maybe you really don't want to go the route you're trying to force your brain to go, but haven't admitted it yet. Possibly, you just need to warm up a bit and think of something else so your subconscious can play around with the idea for a bit.
This is basically what
National Novel Writing Month encourages writers to do – stop focusing on coming up with a final work and just scribble out stuff, no matter how inane, half-witted, lame, or stupid it might be. Somewhere along the way (and you might not immediately realize it's happened), an idea will spark and that image you're trying to envision will start to appear.
Quoth Haskins:
There's a lemur in Sequential Art?
He's a ring-tailed lemur boy whom Jolly Jack was thinking of adding as a neighbor at one point. I know of two images in which he's shown —
this one and
this one.