Not sure what you mean by "won't budge past..."? What happens if you set both (or the 2nd only) at 924Mhz? It does not start at all. Have you increased the core voltage? The currentdraw can, when not tuned carefully, be humongeous at those clockspeeds, so the best advice is to increment one step at a time, and see what happens. E.g., both at 796, then both at 800 etc. It could be that your powersupply (in the usb hub) simply can't deliver the current needed.
Tweaking the pot meters to keep the units hashing at desired hashrate AND at the lowest power (=heat) is fun, but not trivial.