First, computers don't know how to write, but they know how to fix the small stuff. As a result, the minute a piece is all written, and if it's still on the screen, the most reasonable next step seems like checking spelling and punctuation. Second, it's harder to read words from a screen than from paper. Thus your eye gets lazy and doesn't look closely enough to catch minor errors. To catch problems, reading paper is a must .
Revision has two main parts.
If you confuse the two, it's frustrating. You look at the paper as if you never saw it before. For this you trick yourself, forgetting what you thought you wrote so that you can read what the words actually say. Time helps you get this perspective. Use a different typeface, margins, color of paper, any change as long as it makes the paper look new to you.