FAQ #0 - Makuna/NeoPixelBus GitHub Wiki

I am running a sketch that updates the pixels quickly, but they flash or only some pixels light up

If you are using Ws2813 or a newer version of the Ws2812b; you should be using the method specific to these. These newer pixels require a slightly different communications protocol.

Just replace the "method" in your constructor ....

NeoPixelBus<NeoGrbFeature, Neo800KbpsMethod> strip(16, 2);

With an appropriate Ws2813 "method" and see if it fixes it.

NeoPixelBus<NeoGrbFeature, NeoWs2813Method> strip(16, 2);

The Ws2813 methods are compatible with and will also drive Ws2812.

If it doesn't fix it; then you may want to consult this FAQ for more trouble shooting steps.