Photos - Project - Album. Images not showing in pdf preview

Hi,

I created an book album in Projects which I want to print. All images are high res 300 dpi. All images show up in the album edit mode but when I try to save out a pdf to preview, a lot of the images are missing. This did not happen before in the previous OS. I have upgraded to 10.11.1

I also tried creating the duplicate album and added the images again, but still the same issue.

Thanks,