Are you using Windows or Mac for connecting to the Mayfly board? Have you tried more than one USB cable? Are these two unresponsive Mayfly boards brand new or are they running previously-programmed sketches? Are there any LEDs that light up when you power the Mayfly board, either from a USB cable or a LiPo battery pack? Is the small slide switch near the FTDI connector set to the “USB/Lipo” setting and not the “Ext” setting?