Deck is Sagging

The Back Deck has settled through time , we've been here 15 yrs and the Deck was Added before our time.. The Back of the House is L Shaped with the Deck 21ft Long by 16ft Wide.. I would have done things differently , With settling through time the Deck has a bit of a Sage to it Now. The Joyces were done in 2 sections attached (1) 10' L X 16' W (2) 11' L X 16'W.. I would have run the Joyces @ 16' the entire 21' with adequate spacers in between.. The 16' Deck Boards laid across staggered so the seems would be uneven ?? Plus presently there is not enough Pillers to evenly support the Deck Weight.. Thoughts and Opinions Appreciated , I'd add Pictures but Sorry I'm unable to..Cheers Thanks