Your water parameters look good, maybe is the flow. Have you done some change to the water flow in your tank? Maybe changing the position of the powerheads or something.
This is just a thought. Do you think that's probably your nitrates aren't really at zero? I can see in the pictures that you have some hair algae on your rocks, maybe your nitrates are high, but because you have an algae bloom the algae is using the nitrates and that could be the reason your test reads zero. I'm not an expert, but I have read about that in other threads. Hope you can find was bothering your nem. :)