Importing MP4 to Animate Only Imports The First Frame

I've done this sucessfully once, when an MP4/MOV file is embedded into the timeline using the "Load external video using playback" option. I like to sketch in another program to export as a .mp4 and then refine/color in Adobe Animate. However when I try to do this now only the first frame is imported, despite how many blank frames there are/what's selected etc. (if that's the problem). I've also tried converting the video to a .fla in Media Encoder and opening it that way, but the entire video is just white. When importing .mp4's from my other program (Krita), when choosing the skin it does say "No minimum width or height" if that's the issue, but I'm still unsure how to fix this. Thank you!