Every official response I've ever seen in regards to the gem videos is a blanket statement of "The videos are a random promotional offer and may come and go at any time." Some players never have them, some always and others still have the offers come and go. I'll cross my fingers that yours comes back soon.
The other free gem offers (the surveys and whatnot offered through Fyber) is talked about a few posts back on this thread. It also seems to be a situation of just having to wait for it to return.