The center is last part to "cook" so I'm guessing you're not leaving them in quite long enough. I've found my shrinks come out better if I up the temp (around 400) and leave them in about an extra 30 seconds after they get done shrinking. When I pull them out I press them with a bottlecap to make sure they are totally flat.
ETA: I've found that the range of prices for these bracelets are $74 (on sale) to $325! I guess I can see why the details on how to make them are so scarce...
Isn't that amazing!? Fun fact- I have a friend who makes fairly simple jewelry and found that it absolutely would not sell if she priced it UNDER $100. Once she jacked the prices she couldn't keep up with the demand.
I've got that issue of Craft (and a ton of bottlecaps) and was equally bummed at the pretty ring & bracelet with no explanations. Of course no artist wants to detail the steps they use to make their products but c'mon Craft!! Quit teasing me! Next they'll do an article on making your own tire valve covers and accompany it with photos of hand-made cars. ;-)
This weekend I picked up a pack of Tsukineko VersaMagic stamp pads and they worked great with my shrink plastic. They weren't as wet as the Staz-On so I got better definition with more detailed stamps and no smudging after just a few minutes dry time. The colors are really lovely as well.
Staz-on ink works great and comes in a pretty good selection of colors. I've played with regular pigment based inks and they don't dry well, but *if* you can cut them out without smudging them you can seal them after shrinking.
So THAT'S why I'm having all kinds of problems with my clay right now! My Premo is sticking to the pasta maker (and everything else) and I kept thinking, "Dammit, I bought a ton of this because I thought I liked it. I can't believe I got the wrong brand. Grrr... mumble.."
Translucent is the workhorse of clays! Wrap your canes with it, sandwich it between layers of colors for faux effects or mix random crap (spices, colored pigments or even dryer lint!!!) in it. If you don't over bake it should be nearly clear and you can get the most awesome effects out of it.