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? 
While I start mine on the stovetop, it mainly cooks in the oven.  That might help. 