ESP32 and erratic updating problems | ThingSpeak Apps | Forum

I use esp32 and two sensors. The esp32 wakes up from deep sleep every 2 minutes and updates 2 channels using ThingSpeak lib. I keep have erratic updating problems, i.e. one or more successful updates followed by one or more -304 error codes, then good again and bad again...it's been going on for for 2 days without any apparent pattern or cause. Below is the relevant arduino codes:

ThingSpeak.begin(client);

ThingSpeak.setField(1, f1);
ThingSpeak.setField(2, f2);
httpError = ThingSpeak.writeFields(CHANNEL_ID, CHANNEL_API_KEY);
if(httpError == 200){
Serial.println("Data sent successful. " + String(httpError));
}else{
Serial.println("Problem updating channel. HTTP error code " + String(httpError));
}
delay(15500);//free account data rate 1/15 seconds
ThingSpeak.setField(3, httpError); //previous thingspeak return code
//ThingSpeak.setField(4, NoWifiCount);

httpError = ThingSpeak.writeFields(CHANNEL_ID, CHANNEL_API_KEY);
if(httpError == 200){
Serial.println("Channel udate successful. " + String(httpError));
}else{
Serial.println("Problem updating channel. HTTP error code " + String(httpError));
}