Sometimes you will find that certain links in slack do not preview or unfurl inside of a channel or conversation. When you have ruled out all of the basic reasons set forth in the Slack share links documentation setup here here are a couple more that you can look into.
The order of priority form the content that is uses to get the preview information is as follows: oEmbed, OG or Twitter tags (whichever comes first in the page), HTML Meta tags
If the meta tags are beyond 10,000 characters we won’t be able to pick them up – This comes from a direct support chat that i had with them and it turned out I was adding a very long CSS stylesheet at the top of the page and so the meta tags were beyond 10,000 characters.
Check your twitter card validator on the page here
Slack unfurl or preview cache is cleared every 30 minutes per Slack support. (Maybe old reference)
Look for blocked links or domains in the attachments list in the Slack admin: https://TEAMNAME.slack.com/admin/attachments/