Sometimes if you put something padded under your cardstock, you get a better impression than you do on a hard surface. I know theyre pretty obsolete, but try a mousepad if you can find one... Theres the possibility, if that doesnt work, that the stamp is cut unevenly or your block may be warped.
Fun Foam will work, like you purchase at a craft store, if you can't find a mouse pad. I use the thickest fun foam I could find, a lot. It helps.