This happened to me as well and I had to force the machine to show. But you can try to:
1. right click on the collection and click "Update membership" and then press yes on the pop up box.
If that doesn't work
2.Go to > Assets and Compliance > deivices > find the computer you just imported and right click, press "add Selected item" > Add selected item to existing Device Collection > go through the prompts adding the computer to the desired collection, go through til you finish and wait about 3 minutes and then repeat step one on the collection you added your computer into. The computer should show up.
Please reply letting me know if this works.