Did you rinse off the orange before adding? I suppose pesticides could kill it, but I would doubt that just the amount of some zest would be enough. You can't rely on airlock activity to determine fermentation. There are so many different factors that can will cause or prevent airlock activity. Do you have signs of fermentation? Is the mead bubbling? Has the gravity changed? Is there a possibility the yeast was not good? Or was the same batch of yeast pitched for both meads?
If, like you said, it made a lot of yeast in the bottom, more yeast than you initially added, then I would say it's working. More yeast means they are multiplying, so they would still be alive and working.
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