I have a Garmin ANT+ footpod (older model, circa 2011). It works fine in Zwift in normal run mode, but it is not recognized if I try to calibrate it in Zwift. I can enter the calibration mode, select a speed to run, but when I start the 60-second calibration test, it says the pod is disconnected. I can exit calibration mode and start a run and it works fine, but I believe it is about 10% off my actual running speed. I am using a Windows 8 platform (64-bit) with USB ANT+ and USB extension. Thanks!
I had the same issue today with what sounds like the exact same setup, though a newer (2016) footpod.
I get the same issue using iOS with a Milestone Bluetooth Footpod. The Footpod is brand new and is detected and will allow me to run but if I go into calibrate it shows it is disconnected.
I also have a foot pod that works well on an indoor track and a treadmill, but shows 3x my stride length in Zwift - giving me ridiculous speeds.