This might help somebody.
I had a problem after reboot or uploading a sketch whereby the rfm69 often wouldn't function. I then put a loop in the setup() function which polled rfm69.cansend once a second in the hope that this would solve it but with no luck. The solution is below
radio.initialize(config.frequency,config.nodeid,config.networkid);
radio.setHighPower(); //uncomment only for RFM69HW!
radio.encrypt(ENCRYPTKEY);
radio.promiscuous(false); // set to true to sniff ALL packets on the same network
// the next line solved the problem completely
if (radio.receiveDone()){ if (radio.ACK_REQUESTED) radio.sendACK(); }
Bill