Headset help - exocet2 - wits end!

I have a really simple headset on my exocet 2 TT bike that is just refusing to tighten down - loads of play. It's an fsa integrated one and is just two drop in bearings bottom and top, an expander collar, and a cone spacer. Stem goes on top of that and then there's one of those expander bung top caps as it's a carbon fork.
Interesting thing is that when I got the bike s/h a year ago I had the same issue, so took it to the bike shop fearing that the frame might have ovalised or something and the bearings were shifting about. Guy fixed it no bother, solid as a rock, but when I asked him straight up how he did it he was unable, or unwilling, to communicate what he had done.
Now I've had to change a stem and the problem is back - can anyone suggest something I might be missing? Feels like I'm overlooking something really basic, but all I can see that might be out of the ordinary is the cone spacer is quite tight on the steerer.
Interesting thing is that when I got the bike s/h a year ago I had the same issue, so took it to the bike shop fearing that the frame might have ovalised or something and the bearings were shifting about. Guy fixed it no bother, solid as a rock, but when I asked him straight up how he did it he was unable, or unwilling, to communicate what he had done.
Now I've had to change a stem and the problem is back - can anyone suggest something I might be missing? Feels like I'm overlooking something really basic, but all I can see that might be out of the ordinary is the cone spacer is quite tight on the steerer.
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Your new stem sits higher than the top of the steerer tube so that it has room to compress everything and pre load the bearing.
That the steerer bung is fully pressed into and tight in the steerer tube so it’s not pulling out as you try to pre load the bearing.
Stem was definitely proud of the steerer, everything looked correct. Only thing I can think of is that the collar wasn't quite sitting right and needed some sort of manipulation to really set it against the bearing - guess I can figure it out at my leisure.