Do you see your art before you draw?  Is it with your eyes or does a totally kick ass picture pop into your head?  This is a skill and I am convinced that you have to see it to draw it.  Either with your eyes or your head.  Now, I'm not talking about Abstract Expressionism; that's a different beast.  I'm talking representational.  Developing the ability to represent that image which is locked away in your head is quite a struggle.  I have to say that this has been my biggest struggle with drawing.  I can draw, I know I can and I've completed some pieces which I'm very proud of but invariably there is one that comes along that just doesn't look right and it drains me.  You know what I'm talking about?  Something is off and you're banging your head trying to figure out why doesn't it look right!  You try this, you try that and ... it's just not working.  I found that struggle so exhausting that I used to not draw for quite awhile.  I'd play a video game, watch a movie, still hung up and unable to progress.  Now, I am trying to develop drawing exercises to tap into my head and get those images out and it seems to be working.  Here's some tips that I've found helpful:

-  To get the creative juices going, picture a simple shape (box, circle, triangle, etc.) in your head and draw it.  Rotate it and draw it.  You're trying materialize images no one else can see so start simple.
-  Now try to draw something you like.  For me it's superheroes or cartoons.  Whatever your interests are horror monsters, women, skulls, guns, cars, draw them.  Keep them loose.  Thumbnails.
-  When frustration hits because that image isn't coming out right, draw something else you can see with your eyes.  Look out the window draw a leaf, people, a rocking chair, anything that you can see.  By doing this you'll eventually be able to WORK past the frustration by drawing.  What this does is turn a negative experience into a positive one.  Your changing the way you think and your work process.
-  Foggy areas.  Sometimes part of your drawing is going gangbusters but that one are doesn't seem right.  Start loose and tighten.  Use the side of your pencil and move it around.  Eventually you'll lay down a line that works.  I swear sometimes it's not where I think it should be but when it happens it's like a light goes off and someone screams, "That's it!"

Well, this is what is working for me.  Ultimately, you got to do what works for you. I've noticed more images are popping into my head.  The creative juices seem to be on boil and I'm drawing more consistently.  Actually, I find that I'm making more time to draw because I'm having more successes.  I hope you found this helpful and at the very least, interesting.