Hi,
I just got a ledstrip 1m long with ws2812b comtroller.
I try to setup colors but i turns out only the first led turn in the good color.
The next one was shifted and the third shifted again from the second one and only the forth back to the original and so on.
You can see on the screenshot that i ask for 6 red pixels.
You probably have an RGBW LED strip instead of an RGB strip. The RGBW has red, green, blue, and white LEDs so takes 4 bytes of data per pixel instead of 3.
Don't see any examples in FastLED specifically for RGBW LEDs, if you can install the Adafruit NeoPixel library and run the RGBWStrandtest example that should tell you for sure if you have that type LED strip.