How to get a good crema
There are many variables on the journey to a great crema. The coffee should be as fresh as possible - ideally bean to cup in a minute. The grind should be fine for espresso. If it's too coarse the water will pass through too easily and won't extract all the oils. The coffee should be tamped quite firmly in layers so there are no gaps or channels for water to pass through. But not too dense or the water won't get through at all. It's good to heat the portafiter - you can even warm the whole ROK by running hot water through it before making the first coffee of the day. The water should go in just off the boil. And if you slightly overfill the water chamber you can get more pressure - you don't need to use all the water. Once the water is in lift the arms and let the water in. Lower the arms a bit to let the coffee infuse. Then after a few seconds lift the arms and then lower them fully - a good pull is around 25 seconds. Repeat if you want to add more water. If none of the above helps listen for hissing. If air is being released at the top you may need to renew the O ring. Good luck!