uHex doesn't wake up

Hi, I'm working on a project with uHex (DFR0343) and I seem not to be able to have the device wake up after going to sleep. The example code delivered with microPoly library does not work - the device stops responding once it goes into sleep. I confirmed that by adding certain messages sent out via serial port to the original example code, no other changes. Any idea what the reason for that can be? I'm powering the device from the USB UART converter (3.3V).