Hi, I'm not sure about the specific cause behind this for you but I can list a few things that may contribute to it.
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You don't have an updated sitemap.
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The pages/site can be blocked by robots.txt.
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There may be duplicate content across these pages that Google picks up on?
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Javascript issues.
There are many more reasons it could be. Have you ran a technical audit on your site? Do you have access to digital marketers/developers to thoroughly look through this issue for you? A technical audit may flag up one of these reasons and give you the reason why. I hope this helps!
