Since Cubase is very limited to what video formats can be imported, why do I sometimes have problems even importing MOV files, which are supposedly capable of being imported into Cubase?
Also does anyone know a free program to convert mp4 files into mov … but the kind of mov Cubase accepts? 
Quite a big topic @DH1 … 
But, in essence, this problem is often because of the codec used inside the MOV container (not all MOV files are the same.!). You’d think it should work, but some phone camera footage for example won’t be recognised by Cubase’s video engine, for this very reason - even though it’s a MOV file.!
I’ll guess you’ve already scanned the SB ‘Help resource’ for guidance. Worth a thorough read - a lot of detailed/useful info in there.
Video support in Steinberg products – Steinberg Support
Scroll down for info about codecs.
There are lots of free video conversion tools out there for download or online (be careful - don’t just download anything.! Plenty of ‘bad actors’ out there). Just make sure to choose the right options when converting, matching what the Help file recommends (regarding codecs).
There are other (more elegant) solutions - not expensive - if you find you’re doing a lot of this type of work.
Good luck.!