If, your Volumetric Stop it's a hit and miss, just switch to Time based stop. Problem (almost) solved!
I just had an a amazing espresso shot (Err...In fact I just had my third, very predictable, amazing shot of espresso) today!
How? It's easy, Volumetric brew stop wasn't being consistent, so I switched to time based stop instead.
Pros: it will stop, every time, at your profile exact shot time target - what you have to do is to measure your dose and maintaining same grind size, for the same profile (Change, grind size and your coffee volume/weight will change, change your profile, and your shot target will change, which will impact your volume also)
Cons: You have a bit less flexibility (or not, if you keep a written record of the optimal grind setting to each profile)
And of course when you change coffee beans (or when coffee ages) you'll have to adjust your grinding (but you had already to do that anyway).
Very Nice, Now if only our firmware update Fellow Gods, would be Gracious enough to grant us:
Stop by weight (compatible bluetooth scales. Cough...Bookoo scale...Cough) and hopefully the ability to control pressure, on demand, during brew (with the rotary knob). Would be a very, very nice change..