Slightly confused ...

Ok so maybe I missed something, but I just have a small question.

I know the common theory is that Lycans can't change at will during these movies and can only change once there's a full moon. I also know that the theory is that the silver the vampires shoot them with is supposed to stop them from changing/takes away their energy.

So in the scene where Sonja is about to be burned and Lucien is chained up in the chamber, he cannot change to help her because he has the silver arrows still in his back. However when the full moon comes out, he gains the strength to push them out and turn into his Lycan form. I get all that. (Unless I've made a mistake, in that case someone feel free to correct me =))

What I'm a bit confused about is why Lucien didn't just do what he did again, pushing the silver out of him, when he gets hit with the arrows once he's escaped the chamber and is outside. The vampire guards shoot him just before he's able to jump the wall, and just after he does his wolf call to the Lycans in the forest, he turns back into a human.

Why is this? The full moon is definitely still out, otherwise the other Lycans wouldn't of been able to change, and he just proved he could do it a few minutes ago in the chamber. Does someone need to snap off the end of the arrow like everyone seems to do before he can take it out? Did the newly entered arrows take away some of his energy again? Like maybe he got used to the ones that were in him in the chamber and had enough time to regain some energy, therefore he was able to push them out when the full moon appeared?

It's only a small thing, but something that's been bugging me since the movie ended about 10 mins ago =) Anyone's help would be appreciated.