Headset top cover not sitting right on Trek Procaliber

My daughter has a new Trek Procaliber 9.6, which we picked up yesterday. I swapped off the stock stem for a shorter one, but now I cannot get the headset top cover to sit correctly.
If you look at the image below, the top cover has a few mm gap between it and frame.

Image here https://www.dropbox.com/s/k14qrsd5ckq82bv/IMG_8702.jpg?dl=0
Tightening the top cap doesn't close the gap. I didn't disassemble the headset -- all i did was remove the stem and spacers and put the new one on. The fork did start to drop out of the frame and I had to push everything back in place.
Any ideas? Is there a spacer or something not aligned somewhere? I can't get back to the bike shop today and my daughter wants to ride tomorrow. This should be easy as I'm usually OK with bike maintenance!
If you look at the image below, the top cover has a few mm gap between it and frame.

Image here https://www.dropbox.com/s/k14qrsd5ckq82bv/IMG_8702.jpg?dl=0
Tightening the top cap doesn't close the gap. I didn't disassemble the headset -- all i did was remove the stem and spacers and put the new one on. The fork did start to drop out of the frame and I had to push everything back in place.
Any ideas? Is there a spacer or something not aligned somewhere? I can't get back to the bike shop today and my daughter wants to ride tomorrow. This should be easy as I'm usually OK with bike maintenance!
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If your forks dropped out a bit, that may not be seated properly. On my bike the collar has a little cut in it (so it's not a complete "whole" circle), so it's a little springy and needs to be squeezed together to slot back in. Everything else then drops in on top.
It's the split collar on the left in this pic:
https://www.alibaba.com/product-detail/ ... 14661.html
If it's not that, I've no idea!
Yes it has the split collar and that did pop out of the headset bearing stack as the fork started to slide down. I've pushed it back in but its not completely flat. Maybe that is why the headset cover won't sit down far enough. I'll pull the headset items apart and re-seat them and see!
The cut collar is pushed right in and everything is aligned. I've tightened the top cap and the bars turn smoothly and there is no rocking. It is all working as it should, just with the small gap.
Now I'm stumped!
I don't think it needs to be flush, I am pretty sure one of my bikes has a small gap and it's not an issue.
Titus El Chulo 27.5 | 2017
Trek Slash 9 27.5 | 2015 (building)
https://www.trekbikes.com/gb/en_GB/bike ... 6/p/21893/