Why is my whipped cream dispenser stuck?
I have several of these and what you are describing is a common occurrence. 
As cream is either 'whipped' mechanically or put under pressure with a NO2 cartridge bubbles become trapped in the cream to 'fluff' it up. As the now whipped cream is allowed to rest the bubbles will slowly rise to the top and allow the gas to escape. While the pressure within the vessel remains mostly the same the gas is no longer suspended in the cream and does not get driven out of the container as you are expecting. 

(note: do not 'just' add another cartridge of NO2 at this point. turn the canister upside-down and allow the pressure (and some of the cream) to escape (provide a bowl for the cream). Once depressurized you may open the canister and start over, I'm fairly sure you can reuse the cream if you choose)