"mg" wrote:

There are 6 or 8 spiral nails on our deck that will not stay down.
The
boards are flat on the deck and do not appear to be cupped or
warped.
But, within about 3 months of driving the nails down, they are back
up.

Should I pull the nails and replace them with the next size larger
spiral nail?

Should I pull the nails and replace them with coated deck screws?

There is an old expression about fasteners "working" their way out,
probably from expansion/contraction of the wood or from possible
vibration.

Your best chance will be deck screws, just be sure they are rated for
exterior service.

Lew