One more idea for you to try....what are you stamping on? Do you use a stamping pad ? Even a cheap foam sheet or two under your paper will help. It gives it a bit of give so you can stamp more easly. It's to cushion your stamp against the paper because the table top is hard and doesn't give. Do you get what I mean? kinda hard to explain.
Paper and ink makes a difference too but you could try standing up and stamping on your paper. Little extra leverage or oomph! Of course on top of your stamp pad for all this!
Give it a try and see if it helps!