You'll need to power inject more often.
For full 100% brightness, 12V lights will need power injection around every 2 strings of lights connected (100 lights). Highest is probably every 125 lights, but you'd have to splice in a power tee.
I ran into a similar issue, also using WS2811 lights. I have been waiting on an extra power distro circuit board to arrive in the mail and in the meantime, I tried to lower the brightness to all the strands and my last section of lights wouldn't function properly because of the lack of proper power. I tried 50% brightness, but that just wouldn't work still, so until the circuit board arrives tomorrow, I disconnected those in-operable sections.
