Active USB cables allow cable lengths to be stretched to much longer than the maximum length of 5m allowed by the USB specification. However, although the packet timing requirements are met by the repeaters used in the active cables, there is still a significant voltage drop which can affect devices like the RTL-SDR.
Over on YouTube Shaun Dobbie discovered that his RTL-SDR would not run properly on his long active USB cable, and he suspected low voltage. After opening the case on the USB cable head he discovered two pins which allowed for external power input. By simply connecting an external 5V supply from a battery to the 5V input of the active cable he was able to fix the low voltage problem. If you’ve ever found that a long active USB cable doesn’t work then this may be the problem you have experienced. An alternative to this home solution might be to use an external powered USB hub, or buy an active USB cable that already has an external power input like this or this one.