Problems with automatic transmition on CCA (CC243x)

Hi everyone!

I'm trying to send packets with the strobe command ISTXONCCA but the CC2431 doesn't send anything yet. I'm using the following function to transmit on CCA:

char transmitOnCCA(void) {
       long timeout = tickCount + 5; // 5 ms Timeout
       FSMTC1 |= 0x04; // RX2RX_TIME_OFF (mentioned in the datasheet)
       RFST = ISTXONCCA;
       do {
            if (RFIF & 0x40) { // Tx done
                  FSMTC1 &= ~0x04;
                  RFIF &= ~0x50; // delete Tx done & SFD detected Flag
                  return 0;
            }
      } while (tickCount < timeout);
      FSMTC1 &= ~0x04;
      return 1;
}

The fsm-state of the radio is 6 (RX_SFD_SEARCH) when I call this function. It returns always 1 and no packet is sent. What am I doing wrong? Do I have to turn on some interrupts to get ISTXONCCA working? It is nothing mentioned in the datasheet and I haven't found example code.

Thanks in advance

P.S.: The carrier sense threshold is 0x3F (about 23 dBm I think) to ensure the channel is detected as free.