First, check the items on this FAQ: http://forums.luma-touch.com/viewtopic.php?f=24&t=4866
In addition, the times I have worked with people to resolve the issue, it has turned out to be media that was unsupported by the device that was imported and used on the timeline. We don't block all media that is officially unsupported because there are a lot of variations that work even though they are not officially supported and we would be blocking a lot of valuable media.
However, because we have had this issue crop up several times, in the next version you will see a warning if one of your clips is flagged by iOS as one of three things, not playable, not editable, or not exportable.
So if you have imported media (not gotten it from the Photos app) then you could potentially have media that would be flagged and blocked from the photos app (by the way, sometimes they play in Photos app, but won't allow you to edit them or export them). Please check your media carefully and make sure it is supported.
If these suggestions don’t help, if possible we'd like to have you send us your project so we can take a look and try and resolve the problem.