You will see that in the "Monetization" tab there is a "Usage Policy" drop down menu:
If this is set to "Monetize in all countries" then this is the same as when the video was claimed previously, and it means ads will be shown according to the selections you've made in "Ad Formats". If you click the "Usage policy" drop down you will see new options:
PLEASE IGNORE ALL OF THE OPTIONS APART FROM "Monetize in all countries" and "Track in all countries"
As mentioned already, "Monetize in all countries" means that the video is monetized and will have ads shown on it. If you choose "Track in all countries" no ads will be shown on your video and your video will not be monetized.
Claim your Video and Apply your Ad Format Preferences
After you click on "Save changes" at the bottom, your video should be fully claimed. You can always check the status of your videos if you go to your Video Manager page. If you see the green claiming icon it means your video is claimed. If your claiming icon is grey you will need to claim that video!