I have this vocal clip, I want to split it (cut it in two) and use only one part. But it would not split. All other clips would split but this one won't. Yes, it was copied (it is original, but there is a copy of it farther in the song. Delete that copy does not make it split. So I copied original again and the new copy would split but not the original. Now I am just curious why it would not. Thank you.  

Are there hidden take lanes?

An active RegionFX (like Melodyne) will prevent splitting a clip (as well as some other functions), but I am not sure how that behavior works if copied.

Bounce the clip to itself (bounce to clip) and try again

SONAR write-protects the original clip after it's been copied into another format, e.g. Melodyne. If you think about it, this is essential. Just like the way Windows won't let you edit or delete a document that's already in the process of being edited.
 
When you copied the original clip, that copy became a new original with no restrictions.