How to prevent a sagging middle?
Keep things happening!
In the very beginning you (should have) set up a situation - a problem -that needs to be answered/resolved. You (should have) included the right details that cause the reader to "identify" (become interested in) with the character(s) and what they will do in the situation. The question in the reader's mind is "OMG what will happen to them?"
Make the reader care!
Your characters must have goals. Doesn't matter what they are, but you must make your people care passionately about them. If they are flacid or indifferent, the reader will react similarly.
The book is written by setting up situation/problem after situation/ problem as your characters pursue their goals and meet obstacles. Make them care about each one, and the reader will care, too.
The answer is
emotion, the way all people connect.
You're welcome.