Free version works fine so I purchased the Pro version.
Problem - I create a gallery and it appears to work fine but within a short space of time (less than 10 minutes) all thumbnails disappear
from the gallery and it simply doesn't work.

Hello John!

Do you have Photon module of JetPack plugin activated on your website? If so, please temporarily disable it, the problem will be resolved. Our team is currently handling this problem together with JetPack support. We will implement a global solution soon.

Otherwise, if you don't use Photon, please give us temporary WordPress administrator access to your website. Our developers will check the issue and provide a solution soon. You can send the login information using this form .

Don't forget to mention the link to this forum thread in your message.
Thank you! Have a wonderful day!

Hi Agapi G.
That solved the problem. Many thanks

Hey John!

You are most welcome! Please don't hesitate to let our team know any time you will need further assistance. We will be happy to help you out!

In case you have a few minutes, please share your rating for Photo Gallery on WordPress.org. We really appreciate your thoughts about the quality of the plugin, as well as suggestions on improving customer support. Let me provide you the link:

wordpress.org/support/plugin/photo-gallery/reviews/

Thank you so much! Have a good day!