Try updosing. In the 18gm basket I use just over 20gm and in the 22gm I need to put 26gm of coffee in order to get a good shot.
You will need to tighten up the grind also. I dont disturb the tamped coffee by nutating like I do with other baskets...go figure...and make sure that the tamper leaves the tamped coffee parallel with the lip of the portafilter.
It took a bit of trial and error to get consistency but the above did it for me.