what’s your dose/time/yield, your machine pressure/flow rate, what’s your tamp setting on your puqpress/have you tried manual tamping, what’s your machines temperature, Is the clump crusher on your mythos one clean and not damaged, are you wiping the puqpress tamp between shots, is the coffee you’re using well developed and rested?
There’s heaps of variables with coffee brewing so it’s not always easy to pinpoint the cause of channeling. I’ll go through each point as best as i can with what i know.
Dose/Time/Yield: very possible that you could just be grinding too coarse for your recipe which will not create enough resistance to the water, letting it flow wildly. Try keeping your dose and yield the same and keep grinding finer (time will increase) and see if the channeling stops. (if you’re using brew ratios of over 1;2.5/3> it will almost always channel with a mythos due to fines migration and puck erosion). Also your dose could be the cause, if you’re using a 20g basket but using a dose below 19g or above 21g, you could have too much room between the puck and the screen or too little. Aim for a .5 of a gram above or below your baskets size.
Pressure/Flow rate; if your pressure is too high or flow rate too fast, it could cause too much turbulence initially when you’ve started the shot, meaning the water hitting the puck is breaking the surface and causing channeling. You want it to be a gentle and steady flow rather than a heavy stream. You can lower your pressure via your machine pump or you can buy flow restrictors from your go to coffee supplier (if in the UK Coffee Hit will be your best bet)
Puqpress is a great machine but can also be problematic if it hasn’t been set up properly. The portafilter needs to fit snugly in the forks, but shouldn’t require a push or a pull to insert/remove it. Equally there shouldn’t be so much room that it can’t be left hands free. If you’re using VST baskets, try manually tamping with a 58.4mm based tamp.
Temperature; the higher the temperature, the less resistance over time there will be; heat makes things expand, your puck will expand too much too soon and channel (as well as burn your mouth and probably not taste nice haha)
Clump crusher: if you’re grinds aren’t fluffy and look like boulders all clumped together, that could be a cause for your channeling as the boulders will have more resistance than non boulders.
Puqpress tamp: keeping the surface clean will help reduce the chance of uneven tamping.
Coffee: if the coffee is too fresh there’ll be too much CO2 present in the bean and therefore the grinds, meaning there’ll be extra inconsistency and resistance in the extraction and the puck. If it is poorly developed it will not brew well as it will not have ground very well either.
hope one of these helps!