Delta app check-in not working, what can I do?

Things that worked for me:

1. Update the app and restart your phone.
2. Try the website in a browser instead.
3. Check your details. A typo in the confirmation number or name can block check-in.
4. Check the timing. It only opens about 24 hours before departure.
5. Use a kiosk or the airport counter if nothing works, but arrive early.

If it still fails, contact Delta support. Here is a handy Delta Airlines check-in guide with other options.