Cannot drag certain photos I see in iMovie to my videos.. some will and some wont

It looks like the photos I can see when I am in iMovie will not let me drag them into my project... they are in my iPhotos and show up when I pull them up in iMovie... but I believe some of them were taken with my Nikon camera and not all on my iPhone. Is there anything I can do to fix this? Or is there another reason they won't drag over for me. A lot of my other photos and videos are moving fine. Thank you!