If you have iOS 11 or higher, then the videos that your iPhone records are most likely in HEVC codec (aka H265), which is NOT supported by the Steinberg video engine. It’s another “gift” that Apple has quickly forced on users, once again throwing a wrench into the workflow of many people with little notice. HEVC has not been natively supported by many third-party tools until more recently… and Steinberg will NOT likely support it for a while IMO. iPhones used to record natively in H264, which WAS supported. So unless the files have already been transcoded (re-encoded) into H264, depending on how you have exported them, then you will have to manually re-encode them into H264, either as an H264 MOV file or a H264 MP4 file. Hope that helps!
Once you wrap your mind around the various issues of codecs, which can be a big pain in the neck, you’ll get the hang of this, and you’ll know when you need to re-encode something. Bottom line is that for the Steinberg video engine you need to confirm that the file you are using has been encoded in a codec that is supported.
Again, for easy reference for those just reading: