Why won't some faces subdivide while others do?

Hi there, first post in this forum! I’m new to Blender and while I am working on some models some of the faces of my models won’t subdivide while others do. Can someone help me?

Were they the ngons ? If you subdivide an ngon it subdivides its edges not its face.
Had you already subdivided those faces separately to the rest of the mesh ?
Is that the mesh before or after you want to subdivide ?

You haven’t attached or posted a link to your blend file (why not ??) so we can only rely on guesswork.