It is typically a geometry issue, to know exactly we would need to go into your file and enter edit mode and start diagnosing. You can upload your file if you’d like. But the main culprits are usually double vertices, the angles for the auto-smooth are not deep enough, flipped normals, scale not applied, surfaces not perfectly flat, clamp overlap and some others. Here’s a couple of threads where people have run into similar problems:
https://cgcookie.com/community/18808-can-t-bevel-edge-loops-for-grille-insets
https://cgcookie.com/community/18944-face-not-flat
https://cgcookie.com/community/18880-issue-with-screen-edges
https://cgcookie.com/community/18790-issue-with-inset
