I'm at the point where I have to give the truth serum to mother superior but after i drop the original glass, i went out and got the nightshade berries and then brewed the tea over the fire but now when i go back, it wont let me grab another cup. did i do something wrong?

If my memory doesn't fails, you need to make "tea" first.

[spoiler]

Get out.
Put the deadly nightshade in the teapot.
Put the teapot inthe hook over the fire.
Then take the teapot back and return to office.
In the office, use the teapot with the pile of cups.

http://www.youtube.com/watch?list=EL_4UoAU-nFcs&feature=player_detailpage&v=-oENHegEWYg#t=261

[/spoiler]