I'm having the same issue as OP, though I just started using my Netgear C6250 today. Under "Upstream Bonded Channels" I see that channel 1 is "Locked" and channel type ATDMA, but channels 2-4 are "Not Locked" and are channel type "Unknown." Regarding SNR, there are a bunch of them ranging from 38.8-40.1 dB listed next to channels 1-16 under Downstream Bonded Channels. I don't specifically see where to find "Downstream Power Level" or "Upstream Power Level," can you provide more info on that?
Have you power cycled the C6250 to see if it will range for and lock on to more upstream channels ?