I've been refused many times on UA Express* flights when asking for Coke. While I almost never (maybe never) have been outright refused, the reply is typically something like "I'll give it to you if I have any left." Usually they actually remember to stop by my seat on the way back to the galley.

This happens on high load-factor flights. After seeing how many cans they get to begin with, I understand that the FAs do not want to run out.

*: As I see it UA is just a shell for SkyWest/ExpressJet, Trans States, etc. I've heard rumors of these things called "mainline" flights, "Boeing" and "Airbus" aircraft, and "First Class".