I don't have any direct experience with the Neopixel and micro:bits, but the following may help:
Neopixel docs: https://microbit-micropython.readthedoc ... pixel.html
A similar project using the micro:bit and Neopixel: https://lorrainbow.wordpress.com/2018/0 ... -yourself/
I wonder if it might be a power issue. With the Neopixel connected and your existing code, are you able to light up one LED? Then, if you light up more than one, does it still work?
Also, have you tried testing the connection with the micro:bit using different jumper cables?
Please let me know if any of the above helps. I'm happy to pass on any questions or further troubleshooting to our content team.