barouet wrote:I use medium-light colombian beans, about 20 days old.
Well, 20 days is a bit "long of tooth", but that should only affect crema, not the pour so much. Hmm...you have 2 excellent grinders. That said, assuming they are both in good burr alignment, I'd have to say first check under the piston gasket and make sure it's clean underneath. It can get some "crud" under it, which might (not sure of this) cause the basket/PF to not be totally aligned when inserted.
Have you tried another, fresher coffee. Ive pulled 14 gram shots, but I ground finer, to get a bit more sweetness. I would say you may not be grindijng fine enough? I can tell you, it should take a good bit of effort to get a 6psi pull. I don't mean sweat inducing, but a good solid effort. I lean forward so as to use my should and neck muscles.
MOST importantly: Pre-infuse! Maybe around 1.5-2psi tops, it should be slow drips for 10 seconds, THEN proceed to pull...first getting to 6-7psi, then the last 3rd of the pull.....maybe at a weight of 22 grams on your scale (you're going for a 1-2 ratio?), lessen the pressure. This is a traditional lever style pulling profile.
Let us know if any of this works! Also, for grins, make sure there is no leftover grind in your grinders.