I have had terrible luck with USB cables used to extend power leads for R-PIs. No matter how pretty they look, they are all (at least the ones that you would actually want to pay for) cheap junk and have large voltage drops when passing typical R-PI current.
The only solution that I have found that works reliably is to use a step-down converter close to the R-Pi, and feed it with ~12v over the longer distance.
There is a somewhat long thread on the topic here: