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