Hello,
I think Dick stated a good list of things to check! It is not simple to do each of these right, and only one off can give different throws!

Another very good explaination, especially how to use subtrims is that one:

http://www.rcaerobats.net/DualElev.htm

As Bob says, subtrims must be used only to center servo arm, in order to make it parallel to the hinge line. Then they must not be used anymore, concentrate on mechanical linkages in order to make them as equal as you can. Two things, as Dick said, are easily overlooked:

1) Control horn clevis hole, often this is not on hinge line.
2) Control horn clevis height, the two horns should be identical!