I've recently acquired an old Atomic machine (one of the Bon Trading models) that was in need of some serious tlc after years & years of neglect. After getting everything cleaned & descaled etc, i've found that i'm not getting much out of it. By this i mean that if i fill it up with half a litre of water, at best i'll get back about 250ml after it's run through the grounds.
I've tried a few different methods in regards to ground types & how hard i pack them down, and the aforementioned is about the best i can get.
You are supposed to get some steam coming out of the pressure release valve. If so, the pressure should be good.
Have you managed to take the group head out and clean the insides?
Are all the holes clean?
A dirty group head will prevent anything from going through, naturally.
A good test would be to cycle a brew without any grounds what so ever and have a look at how much water you end up with. You'll probably notice it's pretty dirty as well. I usually fill the atomic up with the provided jug filled to an inch below the rim. I will measure the exact volume when I get back home.
I have two Atomics and just picked up a third. In my experience the grind can be too fine. However that would not impact on your ability to deliver water but it could cause the pressure relief valve to pop. Also, if you find that you are not getting a full brew, there may be a problem with the supply tube that transports the water from the base (via steam pressure) to the group head. I would encourage you to pull the thing completely apart and clean it well. If you are really adventurous, you could pull the plug in the base and look inside. That would allow you to see if the supply tube has corroded away (this would be very rare though). Keep in mind that there should be a very tiny relief hole in the supply tube at the filler neck. If all is well, brew a batch without coffee, let it cool, then see if you can brew off the remainder of the water. If you are unable, I would bet that there is a hole in the supply tube.