Jeremy, there seems to be a built in limit to how far you can zoom in for the online mapping. I have guessed, and only a guess, that when you start to zoom below the level where the streamed ESRI imagery starts to get fuzzy, that ESRI cuts off from zooming any further. Even if you have implemented a layer of higher resolution imagery, it doesn't matter, you will be stopped at a certain zoom level, even if you have other layers with more detail.
Does that sound like your issue?
All we can do is convey these issue to ESRI and hope they include them for consideration in a future update.