Many of us, graduate students, PhD candidates, Post-doctoral Researchers and of-course faculties at various institutions in US would have worked on one grant on the other. In the field of Biomedical Engineering and other studies related to health, National Institute of Health is one of the major funding sources. Even after spending hours churning out the most effective grant application, in some occasions it gets turned down. Peer-review of grants one of the most important and a decisive step in deciding whether the grant gets turned down or approved. Here's a short informative clip on how the grants are reviewed. This is what actually happens in a NIH grant review process.

A score around the range of 2-3 gets approved and anything else decides the level of rejection.. Hope this was informative.