Off the top of my head without having enough details there are a few things that you can try.
I am assuming that the files are in the same folder as the other MP4 files.
Lets start with an obvious. I believe the system parses the file (doesn't look at the tags) based on Artists - Track.
Make sure they are named appropriately.
It could be an issue with Codec's (I have had issues with MP4 files for this reason which is why I usually convert them to mkv).
Right click on the file and launch it using "Windows Media Player".
Zenpoint still uses this in the background to play the files. It's older than MP4 and codec can be fiddly.
If this still doesn't help try running a tool like MediaInfo to compare the codecs between the files that work and the ones that don't.
You might be able to use a tool like shark007 codecs to add the correct codec's.
If you need to convert them Handbrake does do a good job and is widely used/endorsed.
The is also an error logs in C:\Program Files (x86)\ZenPoint\DigitalCenter\Logs which might help diagnose the issue.
I won't post the URL's as I have noticed a few spam/scam URL's being posted recently.
In general the tools that I would recommend you have in your toolbox are
