I recently got an espresso machine and have been using it for a while. The one problem I'm finding is I can't get a crema on top of the coffee. Any tips on how to do this? The coffee machine is a good one. Is it me or is it the fact I'm using Lavazza Rosso, which incidentally is a packet I've had open a few months as I was using it with a cafetiere, when I should be using another type?