It looks like you have too many vertices merging. Just lower the merge fresh hold.
Ok, is that in the actual merge box, because I'm sure I didn't see a option when mirroring geometry?(I'll try it)
It's on the right where you have the Channel box thing open. Below the transform, rotate and scale you have your history, in the history click on the mirrow geom node and it'll be in there. Hope that makes sense.
Thanks Justin it worked!!!I've had this problem in the past too and now I know how to fix it thanks alot :D