Start by inspecting your mesh.
As you can see by visualizing the problem in Paraview, your mesh is not as you would expect:
For instance have a look at how to create your mesh with gmsh (You can find a version of this mesh created with gmsh in my github repo).
To convert this mesh to XDMF, and read it into dolfin, consider: Transitioning from mesh.xml to mesh.xdmf, from dolfin-convert to meshio or Converter from GMSH to XDMF (with physical groups)