CR3 files “unsupported”

Hi all, any help on this matter would be massively appreciated. 

I currently own a Canon 1DX3 and transfer my CR3 raw files over form the camera to the iMac and then in to lightroom and I've never had a problem but yesterday after doing this some CR3 files wouldn't work and had "preview unavailable" and even when I've then continued to select them for import it comes up saying "damaged/corrupt/not supported but yet some of the images from the same file worked. 

never had this problem and super confused so any help would be greatly appreciated!