We hear you. Copyright claims are frustrating for us as well since we've worked hard to provide content that is Royalty-free and licensed appropriately - and we don't want this happening to you either.
In short, a copyright claim appearing on content you received from Storyblocks can happen for one of several reasons:
The original artist sold the licensing of their content to multiple parties, and one of those parties has registered the content to a Content ID system when they shouldn't have.
The artist or company that represents them registered the content to a Content ID system when they shouldn't have.
A claimant incorrectly believes they are the only ones with the rights to the music, and have registered it with a Content ID system.
The common thread between these reasons is a Content ID system that exists on platforms such as YouTube and Facebook. These systems aren't smart enough to know you have a license to use the content being claimed.
As a result, YouTube and Facebook rely on content creators like you to do the work of proving you have the rights to use the content, rather than asking the claimant making the claim to prove they own the exclusive rights. It is not an ideal system, and platforms like YouTube have been receiving more calls from the creative community to improve their Content ID practices.