Clips are offline in final export, but are online in the project.

Apologies the photo isn't very clear. But essentially I have a few clips that say that they are offline in the final exported clip (see left hand side of photo) - yet they are online in the project (see right side). 

I  have created a new sequence a copy and pasted, and this hasn't worked. I have also used previews in the export and this has not worked either.