Importing mesh to Unity, losing some of the model

When I import a blend project to unity for testing, some of the mesh is being lost like the back of the character’s head.

I can post pictures if needed, but this may be a common issue that I am not importing the project correctly?
I am simply dragging and dropping the mesh into Unity’s Assets folder and then adding it to the hierarchy of the project.

The mesh is large with many vertices and is getting split into several pieces, but I am making sure to use the main Asset component for the entire character.