Originally Posted by
dabac
There is a problem with your description of events.
On the kind of steerer tube extender you say you've got, I don't think the top cap screw goes into the Star nut.
The extender I had, had a dividing wall at the bottom of the narrow section. On installation, you removed the top cap, which exposed the wall and an Allen screw with a thread cut into its head. Then this normally hidden screw was used to set bearing preload. Then you tighten the clamp screws, slide the stem on and put the top cap on.
This is the type of extender I've seen as well. There's no need for the top cap to be "tight" on extenders since it doesn't set the preload and is more likely to bottom out on something anyway.
You most likely need more spacers below the extender as mentioned by other people above.