Why won't my headset tighten up?

It is a tapered FSA on tapered Rebas that came standard with a Giant before they went to the current daft standard. On most headsets I have had you can tighten them until the bars will hardly turn then back off until you get a reasonable tension. On this one it gets fairly tight with a tiny bit of play but the bolt carries on turning without getting tighter. I changed the star-f nut which made no difference and have checked spacers to make sure nothing is fouling and the steerer can be pulled up by the central bolt but no different. Any bright ideas please?

Got enough gap between the top of the steerer and the top of the stem? Sounds like the top cap is bottoming out.

That was my initial thought but there seems to be enough spacer to stop the steerer touching the top cap.

Stem bolts undone and stem free to move (have to ask)

Does the bolt eventually go tight and stop turning? If not then something sounds stripped or the star nut is moving.

and is the star nut deep enough and again not bottoming out on the top cap?

Does the bolt have an un-threaded section?

If so the star-nut might not be deep enough

You may have over tightened the top cap bolt causing the threaded section of the star fangler to come loose.

There was the problem of the taper being too long and fouling the top race on bikes with short headtubes. Don’t know if the Giants were effected

The bolt does have an unthreaded section but the star nut is fairly deep. I have just been peering down the steerer and there seems to be a ring etched around the tube by the star nut , leading me to suspect the whole nut could be moving which is odd as it is the second nut that has done the same thing. I think I am going to have to look for one of the various devices that go through the steerer to tension the headset.

There’ll be a bit of a scrape mark from it’s insertion anyway.
Take the stem off and see if the top cap will bottom out on the bare steerer.