i've got thesame problem on almost all 3th party cases
make sure the little tab from the lcd cage that holds the umd door closed is on thesame height as the little metal bar on the umd door (on 3th party doors it is a little bit lower so you have to bend the tab from the lcd cage a little bit down)
is the umd door string in the position? there is a little "road" for it on the side of the umd door where it can slide, chceck that
that spring makes sure the umd door keeps open when the tab on the lcd cage isnt holding it closed