Here are the reasons that some files in your iTunes library may not be sent to your iPod:
Software needs to be updated
Download the latest version of the iTunes and then update the iPod..
iPod can't play the type of file
If you're not sure what file types iPod can play, here is the list for you
Compatible codecs
iPod can play songs encoded with these codecs:
* MP3 (from 32 Kbps to 320 Kbps)
* MP3 Variable Bit Rate (VBR)
* AIFF *
* WAV (with no compression)
* The following require iPod software 1.3 or later:
o M4A AAC *
o M4P AAC (Protected) *
o M4B AAC (iTunes Store Spoken Word files) *
* The following require iPod software 2.2 or later or iPod mini 1.1 or later:
o Apple Lossless Encoder
* Requires iTunes 4 or iTunes 4 for Windows. AAC files also require the most recent update to the iPod software.
Compatible sample rates
iPod can play songs with the these sample rates, and with all compatible codecs:
* 8 kHz
* 11.025 kHz
* 16 kHz
* 22.050 kHz
* 32 kHz
* 44.1 kHz
Audible Spoken Word files
iPod Software 1.2 and later can play Audible spoken word files in formats 2, 3, and 4. These files have the filename extension, ".aa".
iTunes 4 and iTunes 4 for Windows work with Audible spoken word files.
Incompatible codecs
iPod cannot play songs that use these audio formats:
* snd
* QuickTime
* MP2
* MP1
* WAV with compression
* WMA and WMA Protected
* any other format not in the Compatible codecs list
If you copy a song that uses one of the incompatible codecs from your iTunes Library to iPod, you can select the song on iPod, but it will not play. Instead, iPod skips to the next song in the playlist.
Media file can't be found
Double-click the song in your library and show iTunes where it is.
Media file not authorized to play on your computer
Double-click the media file in your library and enter your account name and password.
Podcast size exceeds available space on your iPod
If your podcasts are set to autosync with your iPod and there is not enough available space left on the iPod for the current podcasts to transfer, your purchases will not transfer.
Turn off the option to automatically sync podcasts on your iPod and then re-sync your music.
Too many accounts
There is already content from five other iTunes Store accounts on the iPod.
The latest version of iTunes explains most of the reasons why some files aren't sent to the iPod. Get the latest version of iTunes
If you are getting no message explaining why specific content is not being sent to your iPod, you may need to reset warnings on your iPod.
Source:
Some songs in iTunes won't copy to iPod
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