erm i cant think why you still have a gap down the middle.
I assume you have selected all the verts Clicked on the little button as in the image below?
Then using the move tool and holding down x move them all to the middle.
Otherwise i dont know. soz
Im sure someone will come up with a solution though.
I'd do the above, THEN duplicate it. not mirror, then combine, then merge all the middle verts
those who succeed are only the failures that never gave up.