Why did Frodo's mithril shirt not protect him from shelob's stinger in return of the king?

ok so i am talking about the movie i have read the book blah blah blah he gets stung in the neck BUT in the MOVIE why did he get hurt and therefore stung. he was stung in the stomach its very clearly seen by that, but the mithril shirt is indeed on because the orcs apparently start fighting over it. How in the world was he not hurt by this. i ask my dad a million times and he never really gives me a clear answer.and it has been bugging me for a few years.