There are two reasons why you might not be able to share a given post:
1. The post content is too long
For Twitter, if you edit a post and it is more than 280 characters (not including the URL), then that post will no longer be sharable on Twitter due to the social network's character limits.
2. The content is not enabled for that social network.
If the option to share to a particular social network does not appear on a post, then that means your program admin/content creator has decided to not enable that particular social network for that post.
For example, if you do not see a 'Share to LinkedIn' option when tapping 'Share' on a post, then that means sharing to LinkedIn is disabled for that post.