Question: An agent returned my manuscript and said I need "more tension." Any idea what she means by that?
Answer: The term tension, in fiction writing, has to do with the amount of stake the reader has in your characters. The more the reader cares what happens to your protagonist, the more tension there is.
I know I sound like a broken record here, about the motivation thing, but Good Motivation can and does increase tension. Agents have said (to me) that a manuscript with middle sag "lacks tension." If the first turning point is resolved and we're building toward the crisis, but nothing much is going on right then, tension can be increased by inserting action that magnifies the danger to the hero or heroine. And any scene that makes it look like the protagonist is in danger of losing what he or she wants in the story can increase tension.
Anything the reader can identify with that hurts or embarrasses the character can increase tension as well. In one of Dick Francis's titles, his hero, Sid Halley, has lost the use of his hand. He is embarrassed that he can't cut up his food at a formal dinner with his ex-wife, who makes fun of his handicap. That kind of scene can increase tension.
And in Francis's book, he slid the clues to the real killer right into the dinner conversation while I was so distracted I missed them completely.