Check that its app embed is still on in your theme's App embeds: Popups, Slide-ins or Announcement bars, matching the popup's type. Publishing switches it on; it stays off if someone turned it off later.
Check that the popup is Live, not a draft or paused.
Check its rules: the page, the device, the country, guests or signed-in customers, and the schedule.
How often remembers that you've already seen it. Try a private window.
Another popup with a higher priority may be opening on the same page.
A code doesn't work
A shopper's own code only works with the email they signed up with, once, and before it expires.
A shared code must exist as a discount in the market the shopper is buying in. Use Check code under Offer.