Please navigate to the playlist in question in iTunes and check 
whether there's an exclaimation mark in front of the track you wanted 
to add / play. If that's the case, the file behind the entry in 
iTunes' track database is missing or on a network drive that is oddly 
only automatically mounted by iTunes when you doubleclick it in the 
UI, but not when you tell iTunes to do so via AppleScript.

Also, please make sure you have no modal dialogs open (f.ex. 
Preferences or Info) as iTunes will block AppleScript for as long as 
these are open.

Other than these shortcomings of iTunes itself, no other problem 
sources are known to me that could make it not respond to your actions.

Best regards, 
Felix