If you pour a beer with tons of foam, the underlying liquid will be flattened - that's where the carbonation to generate the foam came from, and it's not an unlimited resource.
A week is fairly short to fully carbonate a keg, but otherwise, make sure your CO2 pressure isn't set so low that it's not maintaining the carbonation level in the keg. Ie, looking at this chart
, for your 42°F and a typical 2.5 volumes of CO2, you'd want to keep the CO2 regulator set for around 13 psi. And once the beer is carbed to that volume of CO2, you need to keep the pressure set for 2.5 volumes to keep the CO2 from bursting back out of solution...