Windows 7: Can't add more songs to the new playlist in iTunes

My problem now is with iTunes. I can't seem to be able to add (append) more songs to the new playlist. I can't space down from the last song to copy more songs to the end. I don't know if anyone on this forum is experienced with iTunes, but if so please tell me how to modify a playlist in this way.

Sorry. I ignored the obvious. I had been selecting the last song on the destination playlist before pasting the new songs. What I needed to do was just open the destination playlist and immediately paste what I wanted to add.