[rjett]

As coffee ages, especially in the first 2 weeks, it degasses CO2, thus the one way valves on most coffee bags. Since CO2 has a lot to do with crema, it's usually lacking in coffees that go past that 2 week mark. It really depends on the coffee, but I find grinding older coffees at a finer grind setting may help you pull your shot at similar parameters (input/output/time), but more often than not, I prefer updosing by half a gram or so to achieve balance in the shot. Also, this is probably heresy to any Italian reading, but I don't consider crema a super important aspect of the espresso shot. That part of the shot contains many of the bitter compounds of the extracted coffee, which is why espresso shots are commonly served with a demi-spoon, used to stir the shot and incorporate/dissipate the crema.