Why Sanji is missing so much in the manga?
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.