Some of the problem you're seeing is geometry and physics. Lay a straight edge along the surface of the stamp and see if the stamp is flat. There might be a dish (sunken area) in the middle of the stamp that is causing the imprint to be not as deep in the center. If anything, you want the center of a large stamp like these to be flat or even slightly crowned (higher) than the edges to distribute the pressure of your strike evenly across the stamp.
Another thing is technique. Small imperfections in how you hold and hit the stamp will affect the quality of the impression, even if stamp is perfectly made. This will be more obvious with large stamps like these one versus smaller stamps. Looking at the bar in the first photo, I can tell the stamp was struck harder at the top than at the bottom, either because the stamp was tilted when you struck it, or because you struck it off center. You might experiment with a softer soap to see if you can get a more consistent impression with this large stamp when the soap is more moldable. You might also experiment with several lighter strikes versus one firm strike -- maybe one method works better for you than the other.
All in all, both of your examples look fine to me. For a handcrafted product, the stamps are crisp and nicely colored, they are appealing designs, and they give just the right touch of imperfect perfection.
