About a week ago facebook made some changes that made most link post image previews invisible within comments. I believe this is a new algorithm implemented to fight spam, however, they do sometimes still work. I have been trying to figure out what exactly makes them still work and what makes them instantly regarded as invisible. I have yet been unable to come up with any completely accurate ideas, but for anyone else aware of this new change please feel free to share any information regarding it that you may have. If you would rather not share it with the public perhaps we could message privately and together come up with a workaround. So far I have noted than on fan pages where the posts have not updated in months or more, the link previews work 100% of the time.