Where did you have the back up kegs stored? Seems as if the system set up is fine. The problem probably was that the later kegs were warmer than the initial ones. Next time either keep the back ups as cold as the kegerator temperature or adjust your pressure up one pound for every two degrees. Even compensating for temperature fluctuations will not make you a hero. No one likes warm beer. At least not at a picnic here in the U.S. in the summertime.
Scott Zuhse, Instructor
Micro Matic Dispense Institute