i don't know that it's specific to nirvana. i think a POSSIBLE cause for that other than seeds not being vigorous enough could be that they get too dry to create enough pressure to pop the shells. sometimes too the seeds might just be on the old side. i've noticed that much of the time the seeds that don't want to open have browned membranes that create a dark spot at the tip of the primordae (sp?) right where they'd open.
whenever it happens, i just CAREFULLY stick the point of a sharp toothpick inside the primordae and then wedge them open by pushing against a finger. the trick is to do it with just the tip of the toothpick and not to force it too hard. otherwise, you'll probably snap one of them and lose your plant.
i think once a plant is mature enough, it should be able to pop that membrane on it's own, but i still like to open them up on my own just to give them a quicker start.