Some faces don't show up

I’m trying to retopologize my alien. I can’t get the faces to show up on some parts of the head. Does anyone know a simple way to fix this? Thanks.

Your normals are inverted.
in edit mode, select all faces hit ctrl+N and click recalculate normals.

Could be that you accidentally deleted the faces.
To recreate them , select all the surrounding edges and press F.
Could also be inverted normals.
Ctrl + N for that.