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
good luck
those who succeed are only the failures that never gave up.
https://www.vimeo.com/7080130