Q: playlist doesn't show all songs after syncing

Hello

I bought an iphone and I wanted to sync some songs to it. All my songs are sorted in playlists in iTunes.
I decide to sync 5 playlists. (A playlist of 30 songs)

After the syncing was done, I looked at those playlists, and they were almost empty. (there were maybe 5 songs left)
Also on my iPhone, I had those exact 5 same songs.

All the songs are still in my iTunes library, but it will take me hours to sort them again and place them in the correct playlist.

How can I fix this ? I do have a backup time machine (made a few hours before this problem happened)
How could this happen ? (did I something wrong ? If so; what ?)
How can I prevent this ? (I don't want to make the same mistake)

Is there anyone who can help me ?
(I'm a DJ so this is very important for me)

Thanks a lot !!
Emile VD
Belgium