This seems to be a very common difficutly for beginner casters. You can use a file to file the edges a bit and sometime that will dislodge the connector piece that sandwhiches the leaf between the cast and the sloughed edge. However you do not want to depend on that each time you cast.
I would suggest instead that you can consider this part of the learning process. Your next mix may need to be a little bit drier, when you are patting your mix on it will still pick up great texture, then when you come to the edge of the leaf while putting mix on stay just inside the edge by about 1/8-1/4"; you can use your brush to push the mix all the way to the edge when you are tidying up the back. Others I know stay inside the edge just to insure they do not go over the edge, it is a small portion of the leaf to let go of in the interest of getting everything just right.
I am happy the texture came out beautifuly. Leaf casting is so satisfying!
Happy casting,
Dena

Hi n2dign-

You can use a rasp to file away the stuff that seeped under your leaf (this is a common problem with my castings.)

Be gentle in your rasping to avoid filing through to your cast surface and also, make sure your edges are well supported by sand or some other support to keep them from cracking or chipping.

You could also use a rotary tool grinder to do the same thing, if you have one.

Take it slow, have patience and you will be rewarded.

Jo

If the mix that ran under the leaf edges isn't too extreme, you might try taking a nail, knitting needle or something similar and pressing down on the over run and chipping it off that way. Start toward the center of the leaf where the over run is thinnest and work your way toward the edges. When the edge of the leaf is exposed, finish the edges with a rasp, file or whatever you have handy. A friend uses a tile cutter to gently nip away excess. Welcome to our addiction.