WordPress returns old thumbnail size after changing media settings

I have had a requirement to update a WP build so that the thumbnail size changes from (the default) 150×150 to 500×500.

I’ve amended the size within the Settings > Media and utilised the awesome “Regenerate Thumbnails” plug-in to create the image files at the new sizes. So everything should be cool… alas not!

When I call my new image thumbnail wp_get_attachment_image_src( get_post_thumbnail_id(get_the_ID()),'thumbnail'); WordPress is still returning my image as ...150x150.jpg and not the new thumbnail size.

I’ve attempted updating the page/post in which the image appears but this doesn’t seem to work. Any ideas?

Note: I am using a multi-site instance of WordPress, but I’ve checked each instance to ensure that the images have been resized and that the settings have updated. Everything seems to be working correctly.

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