Choose your coffee grind carefully! There are different grinds, espresso coffee requires a fine grind (finer than the others). Cf Coffee Tips
At first glance, one would think : "not a big deal!". But to tamp properly, there are a few rules to follow.
1) I fill the adapter 2) I tamp (with a tamper or what you have at hand, for instance a bottle cap) 3) I repeat the operation 2 to 3 times.
Be careful not to tamp like a mad!