Just off a brief Skype call from helping a customer trouble shoot
Just mentioning that the press line on the inside of the standard LONDINIUM double basket needs to be visible even for an 18g dose solved his problem – the press line must not be covered or you will have performance issues
Perhaps it will help you too, so here is an image of what I mean.
The above image shows the height of exactly 17.3g of coffee after tamping when using the LONDINIUM double basket which every single owner has for reference
This was tamped using only fingertip pressure, as enforced by the design of the LONDINIUM button tamper
It probably goes without saying, but if the top of your puck is covering the press line you need to grind finer