Q: Not all songs from a playlist are synced to iPod nano

I certainly wouldn't say songs are in their "native" format for iTunes, since I had to download them from youtube and convert for iTunes

Are all of the songs that do not sync these converted songs?  The two that do sync are not these converted songs?

In iTunes, show this playlist and right-click on one of these songs that do not sync, then Get Info.  In the Info window, go to File tab.  What does it say for kind and bit rate?

Also, on the Info window File tab, find location.  Does it give a file path to the song file location on your computer's drive, or does it say iCloud?  If it says iCloud, the song is not stored locally, so it won't sync.  And since these are not purchased songs from iTunes Store, IF they are only in iCloud, they must be in your iCloud Music Library.  If would be important to know if youriCloud Music Library is from a subscription to iTunes Match or Apple Music.

It makes sense that the AAC song syncs.

I think "MPEG-4" is the problem here.  That's not MP3 and it's not exactly AAC either (although related).  MPEG-4 usually describes a video file, not audio file.  These are audio-only files, correct?  MPEG-4 is not on the list of supported formats for iPod nano Audio Playback, according to the tech specs for the most recent iPod nano

https://www.apple.com/ipod-nano/specs/

Since these songs play in iTunes, you can try converting these MPEG-4 files to AAC in iTunes.  First, go to iTunes preferences General pane.  Click Import Settings button.  Set it to Import Using - AAC Encoder, with Setting - iTunes Plus, then OK.

Go to the playlist with these songs, and select the MPEG-4 songs at the same time.  From the menu bar, File -> Create New Version -> Create AAC Version.  In the playlist that syncs to the iPod, replace the MPEG-4 songs with the AAC versions.  Try syncing iPod again.

since I had to download them from youtube and convert for iTunes

The above method (using iTunes) converts an audio file that was already converted.  That may reduce audio quality.  Alternately, you can redo whatever conversion method you previously used, to create either MP3 or AAC audio files from your original source files.  Another way to convert is using QuickTime Player, if you can open and play the original source (video?) files using QuickTime Player; from the menu bar, File -> Export -> Audio Only.  This process creates AAC audio files.

There is actually a setting, on the iPod's Summary settings screen in iTunes, that converts song files while syncing.  Depending on your iPod model, it may say something like Convert higher bit rate songs to [128 kbps] AAC.  However, in your case, since the original songs were in a format that is not compatible with the iPod, I'm not sure it would work.  Also, this setting may also convert songs that DO already sync to the iPod, and reduce their sound quality.  The purpose of this setting is to make song files smaller, to fit more songs on the iPod.  The cost of reducing song file size is lower sound quality.

So, what you did to convert the song files before syncing is probably the better option.

Probably because those "MPEG-4" files are not compatible with the iPod, so iTunes did not make the attempt to sync (or convert during syncing).  As I mentioned, the purpose of that option is to make song files smaller to fit more songs on the iPod.  Not to make items (that are not compatible) compatible.  If it's currently still checked, you should uncheck it so that it does not unnecessarily convert songs (and reduce sound quality).