Powering multiple WS2812B strips from different sources, are all grounds connected?
You will want to tie all grounds together, including the ground of the MCU. 

Remember that voltage is a potential between two points. If you have two different devices with a single common point, that will typically be "ground". That way, the potential to all other points will have the same reference (0V). 
Without a common ground, connections between two devices will not have a static reference.  

Look at LED strip 1. If it doesn't have the same ground as MCU, then the green data wire may have "5V" from data to MCU ground, but what voltage will the strip see? It will have "5V" on data, but the ground is that of the MCU, not the strip, and thus the strip will not necessarily see 5V at all. For in intents and purposes, you could consider it "floating" which isn't really 0V or 5V.