The seventh hole is where things turn around, going from overblows to overdraws, also things are going to get a little high pitched from here on, so it might be worth thinking about a lower harp to practice on. The 7 overdraw is a semi-tone above the 7 blow and a tone above the 7 draw.
How to do it
All those high overdraws are happening in a small space right at the front of your mouth, and could be comparable to a backwards blow bend in terms of mouth shape and tongue positioning. I find that they require quite a definite action to get them initiated.
With the 7 overdraw, think about doing a 8 blow bend but this time drawing in, you might find it easier to first blow a clean note then draw a clean note then go into the overdraw. Try going into the overdraw from both clean draw and clean blow, see if it helps.