Me too!
Hah! I had it happen too a couple of weeks ago, but attributed it to my newbie-hood since this was the first time I changed a 23C road tire. I have Shimano WH-R550 wheels and eventually got the Fortezzas to sit properly after a couple of trial and errors. Once I got the tire all the way onto the rim (with the tube inflated to ~20psi) I gradually worked my way around the tire while using my thumbs to really push the sides of the tire downward into the rim. This seemed to work and thanks to a flat I was "fortunate" enough to re-test this strategy with success...
cheers,
Dan