Are you sculpting or mesh modeling? If you’re sculpting. like they said sometimes you might have to go into edit mode. Sometimes, if a face gets too thin, odd behavior can happen. If you’re mesh modeling, you can also double check for extra verts as well as interior edges and faces that might have been accidentally created and shouldn’t be there. Can’t tell you how many times I was filling holes and accidentally grabbed a vert from the wrong side only to notice odd behavior in the mesh that I missed when filling the hole 😀
