Why do only a small number of songs in my iTunes playlist appear on the playlist on my iPhone after I sync? For example, in iTunes my playlist has 50 songs, but on my iPhone the playlist has 17 songs.

I had the same problem and after much research, I noticed that the songs were indeed on the iPhone, but just not appearing in the playlist - i.e. I could search for each song on the phone and it would be there, but just not in the playlist. It has something to do with the new iOS5 that stores info by something called Album Artist. I had to go into iTunes and go to View > View Options and then select Album Artist so that column would appear inthe iTunes window. Then I would just copy and paste the artist into album artist, resync the phone and the songs would appear in the playlist on the iPhone as well. Weird but works!

Sounds like a problem I just had which I have just fixed. Some of my old MP3 files sync'd okay but some did not even though they were all the same file format.

If they are mp3 files it may be a format issue. If there is a "Syncing symbol next to the ones that did not copy over this probably will be the case" You can then fix this problem by converting the music files that are in the playlist. If you select them in iTunes and right click then select "Create AAC version" they will make a .m4a file which will sync across fine.

I have since found that changing the file format (although works) is not necessary. All you need to do is change any info of the file so that the song ID in Itunes gets recreated and it recognises it to sync it.

Change the artist name, or albulm name or anything on the info tab on the "Get info" option for each song.

Select all the songs you need to update and change a field, even if it is to put in a comment called "updated"

Much simpler than my first suggestion.