Normal coffee vs decaf I have a De Longhi Espresso machine that takes ground coffee. When I use normal coffee (for example with Illy Classico), the coffee is foamy and generally very good. 

When I do exactly the same thing with decaffeinated coffee (for example Illy Dek), the coffee comes out watery and actually quite bad, with the filter getting almost blocked and steam coming out of the side. With decaf, the coffee seems to block the water, making the flow very slow. I tried to vary the quantity and compactness of coffee in the handle to no avail. I also tried to descale the machine, but the decaf remains watery and undrinkable.

Is there anything I'm missing? Why is decaf behaving so differently?
The decaf process alters the grind and other properties in the coffee.

You should first try a more coarse grind.

And don't use a bladed cutting grinder, to much clogging dust, use the grinders with two spaced ceramic disks for better results. Fine tune your grind size.