CCA problem

This problem really puzzles me...

There are 70 transmitters and a receiver based on CC2530.

Each transmitter keeps sending a packet containing a unique transmitter-id every 4 to 5 seconds. The Receiver has to record all the 70 ids asap.

I didn't use CCA at first, the RF sending codes are as follows:

    UINT8 status;

    ISTXON; // Sending

    // Waiting for transmission to finish
    while(!(RFIRQF1 & IRQ_TXDONE) );

    RFIRQF1 = ~IRQ_TXDONE;
    status= SUCCESS;

    return status;

The receiver could receive all the 70 ids in about 10 seconds. (I have tested for about 40 times)

I thought there must be some RF conflicts, so I tried to improve the performance. What I have done is only add CCA in transmitter's RF sending codes.

    UINT8 status = FAILED;
    UINT8 randnumber;

    macInitRandom();
    halRfReceiveOn(); //macInitRandom will turn Receive off

    while(status == FAILED) {
        ISRXON;
        // Wait for RSSI to become valid
        while(!(RSSISTAT & RSSI_VALID));
        ISSAMPLECCA;

        if (FSMSTAT1 & SAMPLED_CCA) {   //if idle
            randnumber = macRandomByte();
            if ((randnumber & 0x07) == 0x01) {    // Probability of randnumber == 1 is about 1/8
                HAL_INT_OFF();
                ISTXONCCA;
                HAL_INT_ON();
                if (FSMSTAT1 & SAMPLED_CCA) {//if idle
                    status = SUCCESS;
                    while(!(RFIRQF1 & IRQ_TXDONE) );
                    RFIRQF1 = ~IRQ_TXDONE;
                    break;
                }
            }
        }
        halMcuWaitUs(500);
    }

With the above modification, the receiver could finish his work in 10 seconds, too. How ever, it would spend more then 10 seconds (up to 60+) seconds to receive all the transmitter-id sometimes. The probability of overtime is about 10% ~ 20%.

The CCA setting is:

    CCACTRL0 = 0xF8; 
    CCACTRL1 = 0x0A;

Is the CCA value too low?

Thanks for any help.