Why won't my bathroom door stay ajar?Everytime you open it it just
closes over but not shut. It's a new masonite door.

Very few houses have perfectly level floors. That is one reason why
doors tend not to stay in place when left open.

If the door is new and just been installed, especially if the hinges
in the door post or the whole door jam has been replace, talk to the
installer, maybe it can be adjusted.  If the hinges were replaced, he
should have hung the door properly.
It could be that the old door would also not have stayed ajar if the
hinges had been new or oiled, but that the bit of additional friction
kept it from closing.

If the hinges were not replaced, and you think that this could be the
case, you might try removing the door and degreasing the hinges.

(Yeah, this is the unprofessional way I deal with household problems.  :)

You could try adjusting to taughtness (tightness) of the screws in the
hinges etc. I think that might work..worth a try... I mean if the top
hinge is pushed out mroe than the bottom, it might combat the
mvoement, depending which way it goes...if you see what I mean.

Or try ungreasing/roughening the hinge joints so they don't move as smoothly.

To make a door stay open, you have to add a shim under the lower hinge
or remove one from under the upper hinge.