Usually, when I want a second drink in C+, I have been able to flag down an FA that is walking by, and a second (or third) has been provided. Using the button could be seen as aggressive but the response was unacceptable. File a complaint.

Perhaps the biggest consistent miss of Delta service, IMO, is inconsistent delivery of C+ service. It is marketed as a separate class at purchase, upgraded into, and should be treated as such.