Hello Mike_Dewsbury! When you try to Stipple your Leaf colours onto your work, is the Acrylic onto which you are stippling dry? There are perhaps a couple of things to try here;
1. Make sure, if you are relatively new (?) to Acrylics, that the paint you're stippling onto is dry, and that way, you can better control the stippling action.
2. Try to use a relatively contrasting stippling colour to that upon which you're trying to achieve the leaf effect; If the background is dark, then stipple a lighter colour or the other way around.
3. The Acrylic paint doesn't have to be very thick for stippling, but the brush you use will be a defining factor; When I try to achieve this stippled effect, I use a fairly stiff brush, with the hbristles splayed a little. I have an old round brush for this, and dry stipple the brush first to splay the bristles, and this can help put many small spots of paint in a controlled area.
Have a go, and come back to me to let me know if this helps.