Transferred Itunes from Old Computer to New Computer. New computer missing 763 songs. How to Fix?

I backed up my Itunes on my old computer to an external hard drive. I hooked up my external drive to my new computer, downloaded Itunes, then imported the music from the Itunes folder into the new computer's Itunes. After the download was completed, the total songs in Itunes on the new computer was 11171. The total songs in Itunes on the old computer is 11934. There are 763 songs missing on the new computer's Itunes. How do you get these missing songs to the new computer's Itunes?