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/