I've noticed that after a post is a few days old to maybe a week old I no longer have the button to resteem it. I'm not sure exactly but seems after a few days old you can't resteem.

You actually brought up an interesting point, I wish they had an option to restemm with comment similar to how you can retweet with comment. Typically when we resteem something it's because we learned something or we feel there's a message worth resharing, it would be nice to be able to add our own commentary or the reason why we're resteeming it.