I've put on several dozen of those things, and rarely have any success with those small letters. I've tried warming things up, thinking that would lower the viscosity of the adhesive on the release film to where it would let go more easily, but no joy. I've tried extra pressure, and that didn't help. I've tried using the point of an Xacto blade to hold the letters in place as the release film pulled away, but that wasn't working either.
I think it's only been twice I successfully transferred that part, and it was years ago when I first started using them. Makes me wonder if the age of the things has anything to do with it. Warmoth doesn't order them very often, so anything that you'd buy today of what's left is probably 4 or 5 years old or better. Adhesives age, which changes their properties.
Sorry I couldn't be more help, but just wanted you to know you're not alone.