If your posts are in the same blog, share the same language, and have identical tags but still don't appear in the related posts section, the issue is likely caused by a multi-word tag.
HubSpot's built-in related posts function can sometimes fail to index tags that contain spaces or consist of multiple words. To fix this, try creating a single-word version of the tag (for example, use "quickwin" instead of "quick win") and assign it to those posts.
Note: After making this change, please allow up to 20 minutes for HubSpot's cache to clear and the changes to take effect on your live pages.