Maybe I am crazy, but I have the feel that either on the navio+ or on the radio the rx/tx pins are inverted.
It was just working after exchanging the pins but I was relatively sure I connected according to the sheets of the radio and navio.
This a big issue with UART that some manufacturers mark Tx and RX on their equipment accordingly to functions of the IC, while some others change them to make user plug in Tx-Tx and Rx-Rx.
Navio+ has Tx and Rx marked according to the functions of the corresponding pins on the processor, not sure about the hextronik radio.
