From recollection, and at least in my own server case, each power socket is doubled up, most likely for redundancy. So if there are 10 sockets, that probably means there are 5 primaries and 5 secondaries. I think this may allow using with some kind of cheap redundant power supply.
That doesn't mean that you're okay, though, simply because you used 8. You have to make sure that you hit each primary/secondary pair. The secondaries are wired to the primaries, but the primaries are not wired to each other. So if you hit a combo of primaries and secondaries, the 2 that you missed might have been a primary/secondary pair, and that column or row might be dead - similar to what you've described.
I would say that your count, 10 power sockets, sounds wrong. Isn't your server a 24-drive case? If so, I would expect each row to have both a primary and secondary power socket, and you should have 6 rows of 4 drives, so 6x2 = 12 power sockets.
In my server, I simply made sure that each row had 1 power cable plugged in.
The SATA spec length limit is 1m or 3.3ft, so there shouldn't be longer cables. At 1m you're already at the limit, so a good quality cable is important. Try not to use longer cables than you really need, 1m sounds really long. Perhaps there's a shorter cable path that you could utilize?