Make sure you are using Vegetable Tanned leather. Other types of tannage will not behave for you, and will not in most cases retain the stamp impressions.
Proper casing is necessary to both swivel knife cuts and for stamping.
A couple other threads that may be of help to you are http://leatherworker.net/forum/index.php?showtopic=64096&hl=+casing%20+carving&page=1
and http://leatherworker.net/forum/index.php?showtopic=66536&hl
Takes practice, looking at others work and examples, and more practice.
Keep at it, it will get better.
Tom