Part of the problem with the 7 blow may be that it doesn't really bend a full semitone. There's only 1 semitone between the draw and blow notes of the chamber, so you can bend the blow note but only to some microtone in between B and C (on the C harp) The issue with 6 draw is probably a combination of factors but from this distance it's hard to say what is most significantly in play. For one thing, "oils ain't oils, Sol". Some harps are just problematic. I used to be fairly quick to suggest people consider adjusting the reeds. I still think it's a valid approach but I'm probably not quite so keen to send you there straight away.
Can you blow bend the 8 on your A harp? Are these the only harps you have?
The real problem may be your embouchure and perhaps calibration.
If you had a couple of higher harps or even another C, you could work out the likelihood of whether a harp adjustment would help.
Try assessing where the harp is at. What I mean is to play each reed carefully, using the most gentle breath possible to activate each reed, and while you do that, pay careful attention to whether either of the reeds in chamber 6 is relatively slow to respond. It could be that one of them is offset a fraction high above the slot. When you are playing a bend on a diatonic, it requires both reeds to get involved so if either one is relatively high it can be a bit problematic to get the bend started, and require a bit of an embouchure adjustment to get it started. I'm tempted to say it takes a "bite" but that is not really what goes on. It's more like a use of the muscles and coordination of that part of the body which swallows (and talks and breathes and hopefully doesn't choke or drown).
In summary, keep at it. Even though potentially frustrating, continuing to try is educating you about using your body to play the harp, as long as you don't make the tempting error of trying to force it. You know how to play a draw bend, so it's just really calibration of movement and examination of how you are using your muscles to initiate the bend. It's the moments of partial success which can guide you. And consider whether the harp might be harder to get on with than it could be. Look into how to check and how to adjust. Go steady at this though. It's easy to do but also easy to mess it up.
I have had the problem myself. When I was working through one of Dave Barrett's lessons on a D harp there was on passage which required me to jump from the 4 hole to the 6 draw and bend it straight away. I hadn't realized I had a problem until I tried that. I think as you do specific bending practice you will naturally strengthen you embouchure simply through repetition and these things will get much easier
Yeah, I was applying some blues riffs to 3rd position to see what the minor sounded like: -4 6 -5 -4. When I put an embellishment in quickly: -4 -6b6 -5 -4 the bend turns out a lot better. It seems that way I'm being gentler and more natural with it.
I'll keep practicing the lower bends to get those improved before I expect better results on the uppers.
I've been giving this some thought, and I realized I rarely use the 6 draw bend. When I do it is usually in 3rd position. Similarly, I occasionally might use it in 2nd position over a V chord. Additionally, if I play 3rd in usually in A, B or C, so using a lower pitched harp which is part of the attraction of playing in 3rd for me. I can use the full range without worrying about being too squeaky. I mainly sing in G through to C, so perhaps it just hasn't really come up.