Loosen the allen bolt. The metal plates may have gotten a bit "stuck" to each other, so after the allen bolt is loose, take your hand and slap the bottom of the saddle.
Getting a saddle "dead level" can be a bit tricky, because as you tighten the allen bolt, the saddle can shift a bit. Tighten slowly, and if the saddle goes off level, stop and level it again before fully tightening the bolt.