I replaced my pressure treated SYP deck after 12 years.
I now believe that the "vast" majority of failure in that
deck was the fastners used in the installation. They were
16p hot dipped nails and "every single one" failed. After
talking to a number of friends, they all have the exact
same problem.

When I replaced the decking(supports were all fine), I used
a system called "Deck Master" which is nothing but a series
of galvanized strips that allow you to screw the decking
from below.

The finish on the deck is great and I really like
not having nail heads that are sitting there waiting to rust
by the puddling water in the nail heads.

I believe this method will greatly increase the life of the
deck.