Sometimes your drawings are not perfect because of a banal mistake. Neither your drawing skills nor unprofessional tools are the mistakes. The problem can be simply your working position. When you are doing a portrait, it's especially important to set your workplace properly and position yourself the way you could easily get a clear vision of a portrait. If you want to know what exactly you need to do here is My Tip #7 "The Right angle".
The secret of the perfect position is the right angle of your drawing board or a piece of paper. It doesn't matter if you are working standing or sitting down you should always hold your drawing at roughly a 45 degree angle. You do a big mistake if you draw a portrait on a horizontal table. Why is that so important? The answer is the perspective. When you are working on a horizontal space the bottom of the drawing will be closer to you than the top, this way you can't reflect the measures of the portrait properly and you lose the realism. Also, you need to remember to keep some space between the reference picture and the drawing. It will help you to get a good perspective on what you are doing and reduce your mistakes.
Now, try to change the angle of your drawing board and you will see the results.