The first Greasse pencil stroke works fine but after I add second stroke I cant see the faces.Why is this happening?
Check normals orientation
It is possible that some faces has negative normals facing to the cam. You should be able to recognize them by their darker blueish color once the "Backface Culling" option is disabled in the 3D view properties panel.
Bsurface take in account the sequence in which you draw each strokes to understand where the normals are pointing, according to the created face index.
With backface culling enabled, Blender will not show the faces who are pointing not toward the camera. Edges remain though, leading to your situation.
I suggest you to select Backfaces and run the Flip normals commmand.