Puck sometimes sticks to the shower screen

[Loucypher]

I always single dose my shots, 18g, but a couple of times when I remove my portafilter the puck is stuck to the the shower screen, I'm thinking it may be my tamping but I believe I am putting the same pressure every time, my new tamper arrived today which is calibrated so I will see how that turns out.
Note: I have a Gaggia classic pro

 

[Musicphan]

I would say that you are over filling your portafilter. You may want to try a larger basket.

 

[shadow745]

You can also try cutting back on the dose a bit, maybe .5 gram and see if that helps. Keep in mind that different coffees can require a different dose/volume to dial in due to roast level, etc. My typical dose is around 19.3 grams, but if I use something like a slightly darker Sumatran I cut the dose back to around 18.9 grams or so as it's less dense and more volume. If I were to use a lighter roast and grind finer I could likely hit 19.6 or so due to a change in density. There is no specific weight/volume to adhere to, just find what works for you and repeat that.