Tension in story telling

I had a really interesting meeting with Claudia this morning. She hit on a key ingredient to writing a good novel, and one I was struggling with as I reached about the halfway mark of my novel. The ingredient I'm talking about is TENSION. I didn't have any trouble with it in the beginning of my novel as I put my character in a tense situation right off the bat. But there comes a part in many novels where things begin to mellow. In mine, it was a part where the main character and a secondary character were beginning to gel. I mean, gelling sounds like a good thing, but not if it puts the reader to sleep. I had actually recognized this problem in my novel before starting my mentorship with Claudia, and tried to remedy it by introducing a whole new tense-filled storyline. However, Claudia pointed out that this thrown-in storyline only detracted from the main story. The tension needs to revolve around that main story throughout. So I've deleted the added storyline (which was no small task since in wound through various areas of the book) and am back to work with setting a few road blocks and setbacks in my character's path in order to heighten the stakes in her main quest...a very interesting challenge!