R H, several different things could have could have contributed to your problem.
First, NEVER attempt stamping on other than flat leather prior to assembly and stitching. Stamp on a good solid surface such as a piece of heavy marble or granite. You didn't mention your sequence, but do not apply any oil or finish to the leather prior to stamping, just water to moisten it.
The depth of your stamping seems a little on the light side, but the weight of the leather has a bearing on just how deeply you will be able to stamp. 7/8 oz. and up will take some pretty deep stamps.
The leather should be cased (moistened thoroughly all the way through and then allowed to come back to almost the original color), then you are ready to stamp and get the best impression. Any place the leather will fold or be wet molded you will lose depth of the impression, therefore you should let the leather dry completely before assembly and stitching. A little trick that might help you out is to "train" the the leather to fold while it is cased, but BEFORE you start to stamp. This method reduces some of the stretch stress which is fading your stamping.
On pouch type sheaths such as you pictured you can also balance the pattern margins so the all the stamping is done more centered on the front side before the fold thereby keeping the stamped area away from the stress of the fold. Keep the inside pattern margin about 1/2" to 3/4" from the center line of the sheath to the left on a right hand sheath and to the right on a left hand sheath, then balance the outside margin so it all looks balanced.
Finally, and this will just take experience, get the feel for the depth of the stamp and the stroke it takes to get it and then repeat, repeat ,repeat.
You are looking for consistent depth from stamp to stamp. Try to get to the point where you are not striking the stamp more than once to get the impression you are looking for. No more tap, tap, tap, just one heavier hit and it's done!
All my contact information is at the bottom of this post. You can give me a call any time and I can walk you through a lot of this on the phone.
Good luck!
Paul
