I'm no expert but going from the piccy it looks like each bit of the bonnet is it's own piece.

Even though you have combined all the bits, the easiest way to fix this would be to either:

a: with all the pieces combined, marquee select the vertices that appear in the same place (the ones at each corner basically where the pieces join together) and then merge them (edit polygons/merge vertices).

If you open the channel box/layer editor window and then marquee select the vertices that appear in the same place it will display the cv's that were selected. There will probably 2 or more shown in the window.

b: remake the bonnet using only one polygon that is subdivided or extruded etc to get the shape.

Once you have done either a or b do the polygon-sub d conversion.

You should have 1 piece that may have rounded corners and 'soft' lines where the shape of the bonnet is. After that you will probably have to use the select by component options (points, lines, faces etc) and the sub div drop down menu to make the edges and vertices full crease/partial crease or uncreased. The corner vertexes will probably need to be made full crease to get the sharp corner back. Playing with these will then give you the final shape you want.

Make sense?