We all here about it when we're writing. If a scene is dragging on, and the reader's head has dropped to the table with drool dripping out of her mouth, then it's safe to say we have a problem.
So how can we tell if there's enough tension in our stories? We can look at the conflict. Conflict is opposition between characters or other forces that drives the plot of a story forward. Simply put, things need to be happening in our stories.
More importantly, things need to be changing.
Look at each scene in your novel. Where (physically or emotionally) is your protagonist at the beginning of the scene? Where is she at the end of the scene? If she's sad at the beginning, she needs to be happy or even sadder at the end. If she doesn't have the information she needs to find her sister at the beginning and that's what she's trying to get, she needs to figure it out or learn something significant to obtaining her goal.