Open up...
Most players tend to close their embouchure down as they move up to the high end of the harmonica: this causes the reeds to commonly flatten, sound airy, squeal or not sound at all, especially the 7 and 8 draw. When moving to the high-end OPEN your embouchure by dropping your jaw, tongue and opening your throat, as well as keeping your lips as wide as you can and still keep a single note sounding. Think warm air passage... like you're fogging a mirror with your breath. This will ensure big sounding high-end notes.