If you already made the unification and left out some nations, that's it. No more unification events for you. Only a military solution remains.
Otherwise you'll have to start the unification play manually. If you wait for some event to happen, you'll wait until 1936 and nothing will ever happen.
Overall, the best way to do it (and fast at that) is to decline every offer to join with someone, and just build up your relations to +50. Invite them to your customs union, befriend everyone, especially France, Russia and GB, though the latter doesn't matter that much.
Anyway, once you're friends with most and the three GP's, you launch the leadership play against Austria, so that they're forced to give up their unification candidate status. If you can manage to get France and Russia on board (and GB as an extra), Austria will usually give in without a war, which makes you the only unification candidate for free.
At that point everyone with positive relations will support you when you launch the unification. Everyone not supporting you is your enemy (which, at that point, shouldn't be more than one or two small city states, like Oldenburg). Launch the play, defeat your one or two enemies with the help of your 20 allies, and at the end you get a fully united Germany for under 10 infamy in the early 1840's.
Extra points to you if you somehow managed to get Austria to support you, then they'll also become part of your new Germany.
