What you want to do is find out what the Manufactures specs are when it comes to joist spacing, make sure your looking at the specs for the width and thickness of the boards you have. Some call for 12" on center although 16" on center is common practice.

-If the product calls for 16" on center, and that's what you have (triple check), call the manufacturer, but before you do be prepared to get the 5th degree (is there any heating ducts running under the deck, proper ventilation, heavy loads on deck, use a grill on the deck, etc).

-If it calls for 12" on center call your Contractor (be easy on them though, because like I mentioned 16" on center is common)

As long as the wooden structure is sound, a sagging deck with an above-ground foundation-block system can be brought back to level fairly easily by simply jacking up the structure immediately be- side each settled post, adding patio slabs or pressure-treated wood spacers beneath the foundation block, then easing the structure back down and checking for level.

Margery, the decking is sagging not the deck structure!! Guarantee the decking is not set properly or the joists spacing is not correct.