many files didn't transfer from my old MacBook Air to my new one.

Consider using these instructions: OS X: Manually migrating data from another Mac

dwb's reply in this Discussion may be helpful: migration assistant some files didn’t transfer. I never tried that utility though.

Since your Mac is new, you can also avail yourself of Apple's complimentary 90 day telephone or chat assistance. If you didn't already get an email from them describing how to set up an appointment, start with Contact Support and click through the options that lead to establishing personal contact. That's the kind of question they're happy to answer.