DVD Studio Pro 4 final DVD will not play in all players

Hello!

It seems that I am doing nothing out of the ordinary... we've burned a hundred of different projects this year. Final Cut > Compressor > DVD studio Pro. Got a complaint that a clients DVD would not work in their player. We checked and it's true. Only select players. We can't burn DVD's as we used to... all of a sudden. I checked all the preferences... everything seems to be fine.

Any thoughts?