An ad may not appear correctly on mobile phones if it:
- A) is in Lightbox format
- B) uses HTML5
- C) isn’t optimized for mobile
- D) uses Flash or contains large images
Explanation: To make sure your ad looks great on the small screen, you can create a mobile-optimized ad with a message, display URL, and landing page specifically for mobile devices. To do so, select the “Mobile” checkbox next to the “Device preference” setting. If your ad groups contain ads that are eligible to appear on all devices and mobile-optimized ads, only your mobile preferred ads will appear on mobile devices.
You can read more here: https://support.google.com/adwords/answer/2472719?hl=en