The deck was replaced in November 2003, so it's approaching 20 years old. It is made of ipe (iron wood), on a pine or fir under structure. The ipe deck boards are attached by what are supposed to be brass screws, looks like #8 screws with a small T-15 Torx head; one site says they are "Finish/Trim" screws.
I've notice that some some of the heads have backed out and are sticking up; some more than others. The worst seem to stick up maybe 1/8 to 1/4 of an inch. Trying to screw them back down does not work -- they just spin in place.
Investigating why, it seem that the screw goes thru the deck board and into the underlying joist. The smooth upper shaft of the screw is in the deck board and the thread goes into the joist. The joist wood has deteriorated and so the thread has nothing to grasp, which is why the screw can pop up. Also in some cases the screw has broken and the top half is disconnected from the threads.
So the solution seems to be to remove the old screw and move it, or a replacement to another position not too far from the original position, where it can find strong joist wood to hold the deck board in place.
Putting in a new screw involves:
(1) drilling a hole for it. The ipe is very hard. I used a 1/8 inch drill bit.
(2) installing the new screw. At 2 inches long, these take quite a bit of screwing, so the easiest way is to put the T-15 Torx bit into a power screwdriver (I used my power drill with it's variable speed, on slow and easy).
Today I only did 4 replacement screws. Took about an hour.