Why Sanji is missing so much in the manga? An announcement came at the annual One Piece Super Stage Event last December that promised to change all that. The staff and Sir Eiichiro Oda assured fans that Sanji will return in 2016 with an origin story. Sanji's voice actor Hiroaki Hirata will also return. But still nothing for Sanji he is still missing.

Why this will be? Is it possible that he is a weak one amoung straw hats?
TL;DR: Sanji is busy with other stuff, but the next big arc is about him.

It is very simple why Sanji went missing.

Chapter 812:


  Sanji is told, that he has to marry some chick and he has to leave the straw hat pirates.


Chapter 814:


  The crew gets a letter from Sanji, telling them that he has to meet with that girl.


Chapter 822:


   The crew starts its journey to save Sanji.