What is wrong …?

I tested your code and it does display the thumbnail on my testing box. What versions of the plugin and WordPress are you using?

Does your theme support thumbnails? (check this link) Have you checked this feature with one of the default themes?

I just remembered that it HAS been working. So I tried to deactivete plugins.
It turns out that this plugin does not agree with CM Super Tooltip Glossery.

That is not good!

Am I bound to get another plugin, or is there a another way?

I just created an issue on the CM Super Tooltip Glossary support forum:
https://www.ads-software.com/support/topic/thumbnails-not-showing-22

Hopefully we’ll get to a solution so you can use both plugins.

Great service!
Thanks a lot!

I through out the CM Super Tooltip Glossary plugin and got another. S? now everything is working.

Thanks! I’ll continue the discussion with the CM Super Tooltip Glossary devs to see if we can make both our plugins work together.