I don't think 2-3 week old coffee is doing you any favors. As coffee get's longer in the tooth, the tougher it will be to get good pulls. Generally speaking, you have to start grinding finer and dealing with shorter pulls as the coffee creeps up on 2 weeks.

Your coffee is past it's prime. I believe you would have much better luck with some good fresh roast.

Also - I would limit the PF knocking to just once in order to get the grounds a little more manageable. Knocking is gonna cause fines to migrate to the bottom of the basket and an uneven distribution of grounds.

Sorry to be giving conflicting information/guidance. I'm not trying to say Marshall's technique isn't good, but his equipment might be a little more forgiving than a Silvia.

cheers,
~j

Older beans work better with a coarser grind and dosed up.