When expecting a gallery, do you just see the shortcode on the frontend like this?
Then your shortcode does not get processed. The reasons behind that could be:
- JIG is disabled. Check JIG on the plugin list of the WordPress Dashboard, it should be active, not only installed.
- Shortcode processing is disabled for the content area you are using. In case you use JIG in a theme shortcode, excerpts or widgets, check if other shortcodes are working. If they don't, then it's a problem coming from the theme. Make sure the content output goes through the do_shortcode(); function. Check your theme files. Ask the support of the theme why shortcodes aren't processed in your particular area.
- If you copied an example shortcode from a website, such as the old version of this site, make sure there is no "anchor" icon in the shortcode name. It was a hidden link to avoid converting example shortcodes into working galleries.
- If you use any caching plugins, perhaps a page got cached while the plugin was inactive. Caching plugins may only automatically purge the cache when the content is changed on a given page. They may not purge the cache for all pages just because you activated the plugin. Clear the cache of your caching plugin, if applicable.
- Are you using a "Shortcode alias"? That's how you use JIG through a customized shortcode handle. Make sure you set that alias in the settings of JIG, and made no typos where you use it. The justified_image_grid handle is always available regardless of a custom alias.