There are a number of reasons as to why a link preview might not be created when you add a link to the Compose Box or when you're bulk importing messages.
1. The site has blocked hotlinking.
Some sites prevent other sites from retrieving content from their site as this increases the amount of bandwidth needed by the site being linked to. This then prevents our system from being able to pull through the information needed to create a link preview.
This is rare with smaller sites as sharing of links and content is a way for their site to be promoted however, some large sites, especially news sites, can block this as the amount of bandwidth needed to support this is very high.
You can use tools to check if the site you're linking to is hotlink protected by using a site such as http://www.htaccesstools.com/test-hotlink-protection/
2. The link you're including is a link to a social site.
Similar to the above, most social sites do not allow you to link back to a post or similar on a social site. Some social sites such as Facebook can block these messages from time to time however, most will post the message but, as they generally do not allow us to retrieve the information in order to populate the link preview, it's unlikely a link preview will be included in this case.
3. The link is to a document not generally supported by social sites, eg. a PDF.
Most social sites do not support document types other than images or videos so, even if you have a link to this type of document that can be accessed by anyone, unless there is HTML information or similar that our system can use to pull information for a link preview, which is generally not the case with documents, link preview are generally not going to be possible in this case.
You should be able to create a custom link preview from an attached image as outlined on this page, however, we cannot guarantee that this will work either.
4. You're bulk importing messages but have not set default link preview options on the related service in Sendible.
Due to the amount of time it takes to pull through data and populate link previews, the system only creates link previews by default for the first 15 posts if you do not have the option set on the related posting service directly for link previews to be created. You can set Facebook Page posting and Google+ posting services to automatically generate link previews, for example, and in these cases link previews will be included in all posts you've imported which will be sent out via these services.