New kegs that are tapped and then are flat normally will result in a bad seal around the valve in the keg. What causes this is keg handling between the brewery and retailer. Normal wear and tear also is a cause of a bad seal. The beer in the keg degasses and you end up with flat beer. The brewers' attention to specifications also tend to determine if flat beer is sent out from the brewery.
If this is not the case, then your pressure is low and you degass the beer every time you pour eventually resulting in flat beer.
Scott Zuhse, Instructor
Micro Matic Dispense Institute