I'm noticing some of the MP4s I try to import only use the audio
Which version of Hitfilm are you using? 16 or earlier?
You should download MediaInfo (it's free) and create a TREE report for the problem video file and paste it here.
Chances are you have h.265/HEVC encoded video in a "Mp4" file. Confusingly, HEVC is NOT "Mp4."
Hitfilm 16 does support h.265, but Hitfilm Express uses the OS encoder. Microsoft removed the h.265 encoder in 2018. If your machine is running Windows 10 from 2018 or later you'll need to download the HEVC codec from the Microsoft store ($0.99) or use a third party program like Handbrake to transcode the h.265/HEVC file to h.264/AVC/Mp4.
"Chances are you have h.265/HEVC encoded video in a "Mp4" file. Confusingly, HEVC is NOT "Mp4.""
Yeah that was definitely it