Why can’t I transfer some music from iTunes to my iPad? It shows a red circle with a slash through it.
My first guess would be that the song is in a format not supported by the iPad. Right-click the song and select "Get Info" take a look what file type it is and make sure it is a supported format
* from the iPad specifications:
Audio formats supported: HE-AAC (V1 and V2), AAC (8 to 320 Kbps), Protected AAC (from iTunes Store), MP3 (8 to 320 Kbps), MP3 VBR, Audible (formats 2, 3, and 4, Audible Enhanced Audio, AAX, and AAX+), Apple Lossless, AIFF, and WAV
Try enabling manual management, which allows you to transfer individual tracks instead of syncing your entire library:
This is also necessary if you want to use your iPad with different iTunes libraries on multiple computers.