There are a few reasons AdSense Ads disappear from a Hub.
Sometimes people will turn the ad setting to off while in edit mode. There are three settings. High, Low, none.
The other reasons are related to AdSense. Here are a few of them.
1. AdSense may not have ad inventory to display. The budgets for the ads that match your page have been used up, so they won't serve more ads until someone is willing to pay for them.
2. There may be some content in your Hub that is triggering an AdSense filter that is signaling to them that the content violates their ToS, so don't show ads. Adult content, and other sensitive topics are the usual suspects.
3. There can be a brief period of time from when a Hub is first published to when AdSense can properly match ads. During this time, it's not uncommon to see ads appear and disappear for several hours if not a day.