CC1101 Fixed length OK - Variable Length No reception

I need help on this one.

I have two custom identical AVR88+CC1101 boards that I'm making an Arduino library for. I have one board tx'ing "Ping" to the other, whereafter it replies "Pong". Everything works fine, when I use fixed packet length. 
My tx packet on board 'A' (which has its ADDR register set to 'A') is:

uint8_t txbuf[5]={'B','p','i','n','g'};

And on board 'B' (ADDR='B'):

uint8_t txbuf[5]={'A','p','o','n','g'};

The radio settings used when using a fixed packet length are seen below.

When I want to use a variable packet length I only change two registers:
PKTLEN =0xFF
PKTCTRL0 =0x05

And the tx buffers becomes:

 uint8_t txbuf[6]={5,'B','p','i','n','g'};

And on board 'B':

uint8_t txbuf[6]={5,'A','p','o','n','g'};

However, nothing is ever received on board 'B'. The GDO0 pin is never asserted, so I guess it never receives the sync.

My transmit function looks like:

void CC1101::transmit(uint8_t *payload, uint8_t len)
{
  setStateIdle();
  flushTxFifo(); 
  delay(1);
  spiBurstWrite(CC1101_TXFIFO, payload, len); 
  delay(1);
  setStateTx(); 

  delay(25); //Debug: allow time for TX to complete
}

Again, it works fine when using fixed packet length

Any suggestions?

// Rf settings for CC1101
RF_SETTINGS rfSettings = {
    0x29,  // IOCFG2              GDO2 Output Pin Configuration
    0x2E,  // IOCFG1              GDO1 Output Pin Configuration
    0x06,  // IOCFG0              GDO0 Output Pin Configuration
    0x47,  // FIFOTHR             RX FIFO and TX FIFO Thresholds
    0xD3,  // SYNC1               Sync Word, High Byte
    0x91,  // SYNC0               Sync Word, Low Byte
    0x05,  // PKTLEN              Packet Length
    0x0E,  // PKTCTRL1            Packet Automation Control
    0x04,  // PKTCTRL0            Packet Automation Control
    0x00,  // ADDR                Device Address (Overwritten with specific value on each board)
    0x00,  // CHANNR              Channel Number
    0x08,  // FSCTRL1             Frequency Synthesizer Control
    0x00,  // FSCTRL0             Frequency Synthesizer Control
    0x21,  // FREQ2               Frequency Control Word, High Byte
    0x62,  // FREQ1               Frequency Control Word, Middle Byte
    0x76,  // FREQ0               Frequency Control Word, Low Byte
    0xC8,  // MDMCFG4             Modem Configuration
    0x93,  // MDMCFG3             Modem Configuration
    0x93,  // MDMCFG2             Modem Configuration
    0x22,  // MDMCFG1             Modem Configuration
    0xF8,  // MDMCFG0             Modem Configuration
    0x34,  // DEVIATN             Modem Deviation Setting
    0x07,  // MCSM2               Main Radio Control State Machine Configuration
    0x3F,  // MCSM1               Main Radio Control State Machine Configuration
    0x18,  // MCSM0               Main Radio Control State Machine Configuration
    0x16,  // FOCCFG              Frequency Offset Compensation Configuration
    0x6C,  // BSCFG               Bit Synchronization Configuration
    0x43,  // AGCCTRL2            AGC Control
    0x40,  // AGCCTRL1            AGC Control
    0x91,  // AGCCTRL0            AGC Control
    0x87,  // WOREVT1             High Byte Event0 Timeout
    0x6B,  // WOREVT0             Low Byte Event0 Timeout
    0xFB,  // WORCTRL             Wake On Radio Control
    0x56,  // FREND1              Front End RX Configuration
    0x10,  // FREND0              Front End TX Configuration
    0xE9,  // FSCAL3              Frequency Synthesizer Calibration
    0x2A,  // FSCAL2              Frequency Synthesizer Calibration
    0x00,  // FSCAL1              Frequency Synthesizer Calibration
    0x1F,  // FSCAL0              Frequency Synthesizer Calibration
    0x41,  // RCCTRL1             RC Oscillator Configuration
    0x00,  // RCCTRL0             RC Oscillator Configuration
    0x59,  // FSTEST              Frequency Synthesizer Calibration Control
    0x7F,  // PTEST               Production Test
    0x3F,  // AGCTEST             AGC Test
    0x81,  // TEST2               Various Test Settings
    0x35,  // TEST1               Various Test Settings
    0x09,  // TEST0               Various Test Settings
    0x00,  // PARTNUM             Chip ID
    0x04,  // VERSION             Chip ID
    0x00,  // FREQEST             Frequency Offset Estimate from Demodulator
    0x00,  // LQI                 Demodulator Estimate for Link Quality
    0x00,  // RSSI                Received Signal Strength Indication
    0x00,  // MARCSTATE           Main Radio Control State Machine State
    0x00,  // WORTIME1            High Byte of WOR Time
    0x00,  // WORTIME0            Low Byte of WOR Time
    0x00,  // PKTSTATUS           Current GDOx Status and Packet Status
    0x00,  // VCO_VC_DAC          Current Setting from PLL Calibration Module
    0x00,  // TXBYTES             Underflow and Number of Bytes
    0x00,  // RXBYTES             Overflow and Number of Bytes
    0x00,  // RCCTRL1_STATUS      Last RC Oscillator Calibration Result
    0x00,  // RCCTRL0_STATUS      Last RC Oscillator Calibration Result
};