Art isn't like flipping burgers or putting widget A into widget B.... it is different every single time (visual, music, writting, whatever).
Drawing you have to be aware and try to use sooooooooo many different variables (lighting, contrast, color, positive and negative space, balance, anatomy, edges, texture... and much more). At the same time, you have to do it differently. You can't simply use what used for you yesterday (or, well, everything ends up looking the same).
What you see in an artists portfolio isn't what he / she does each time he /she sits down. It is the best of the best that has been done.
Often what you see is only the "stuff that works". But what you rarely see are the piles of rejects they have as they tried to work out that good stuff (or, often, instead of piles or rejects they just keep going back and fixing all the stuff that was wrong with the piece, revision after revision).
The thing is, those rejects are actually pretty useful. I sometimes end up drawing a HORRIBLE creature... but... actually like one or two of the ideas I had. So, the next one, I take those good parts and move the idea forward (without that crappy reject, the final wouldn't have been as good).
Basically, art is problem solving. It is never a case of "here is the solution". It is always a process of working towards figuring out that solution (from sketching, silhouettes, color tests, references, blocking in, adjusting....).
Oh, and sometimes, what "makes a piece work" isn't about what is really the important part (like the CREATURE in a creature concept). Sometimes it is just a matter of a different color or just adding in a better background :|
Andddd sometimes, when looking at a picture for a while, you stop "seeing" it. You miss completely obvious things or think something works when it is crappy. Which is why it is sometimes a good idea to go take a break now and then When schedule doesn't give you that break, well, you just have to keep going and miss the obvious (hopefully someone with more time catches it :|