Perhaps you don’t apply yourself consistently throughout your essay. I suspect you start off ok but don’t complete the job with the same skills.

Perhaps your ideas peter out. Perhaps you have some brilliant ideas alongside mediocre ones.

Perhaps you get sloppy with technical details, such as spelling and grammar.

THE GOOD NEWS IS THAT it sounds like you’re doing a good job in certain areas. You want to extend those skills so that more of your essay reflects your potential. There are many fellow students who can’t touch your hem when you are really on. You must appreciate yourself.

Do talk to your teacher. You have a talent and you don’t want to bury it.

Consistent flow doesn’t just mean from paragraph to paragraph, it means from idea to idea, from sentence to sentence. Every idea, every word, should not be sitting on its own little island, it should be part of an interconnected stream of words that take the reader on a smooth ride from point A to point B. Often overlooked, transitions are the easiest and most effective way to give your essay that even feel. Here’s an illustrative example of poor flow:

http://www.mommd.com/outstandingessay.shtml