Why are actions like boosting & liking unavailable on some timeline items?

It all depends on how you added that feed via the FeedFinder. 

If you created the feed using the connectors listed in the six examples above, there's no personal account associated with that feed and so actions are unavailable. This is a great way to monitor the content of some social media feeds but it also means Tapestry can't apply actions since there's no account associated with the feed. In order to boost or like those feeds, Bluesky or Mastodon needs to know who is boosting or liking the posts.

The workaround is to use the "open in" icon (arrow pointing into the square) to open the item in the third party app of your choice. You can then like, boost or bookmark from there. Alternatively you can follow that person directly in Bluesky or Mastodon, and login via Tapestry in order to boost/like that post.