Hi Benoi31,
Did you have a heavy base directly under the stamped surface? If not, it's the most likely reason for unsatisfactory results.
I bought the cheapest set I could find and it seems to work fine. The only thing that's not great about it is that some letters' stamping surface aren't exactly perpendicular to the letters' shafts. Which means you have to hold the stamps and hit them "at an angle" which is not so cool.