I emailed customer service about a problem 3-4 weeks ago. Rather than the typical email, they actually called me and said they'd look into it, but I haven't heard anything in quite some time. What's the best way for me to follow up?

(For those who are interested, it was a missed upgrade (rare dinner flight for me) where I was #2 on the list, the app and @DeltaAssist showed it going out with 5 empty seats up front, but the FC cabin was full; the gate agent was pulling people from the back as I boarded.)