There can be different factors in play here. When you say USB 3.1 do you mean the Type-C connector?
What end are your connecting to your computer? The Type-A [rectangular] connector has two rows of four pins, if you don't put in fully into the port, only the front four pins will be connected and the cable will act like a USB 2.0 cable. USB 3.1 (Type-C) clicks all the way or it won't work at all.
I thought they were both the same.
Bought an usb 3.0 cable to connect a ssd.
Best speed I could get was 28/30MB/s.
Bought an usb 3.1 cable and speed went over 300MB/s.