Problems receiving (all) LoRa Packets - esp32

Question

At the moment I try to send packets between one Heltec WIFI LoRa V2 and another by reading the serial-line and sending the input via LoRa.
Small packets (like 30 bytes) work every time, but as bigger the packet gets the packet won't be received every time or even never.
So I write a little sending loop, where my sender sends at every iteration a packet, which gets every time 10 byte bigger, and surprisingly every packet was received by the sender (I tried that until 500 bytes).
After that, I wanted to send a 80 byte serial input message and this did not work. Do you know what's the problem with that?
void setup() {
// ... LoRa.begin(); ....
LoRa.onReceive(onReceive);
// ... LoRa.receive(); ...
}
void onReceive(int packetSize) { // uses the interrupt pin on the dio0
String packet = "";
packSize = String(packetSize,DEC);
for (int i = 0; i < packetSize; i++) {
packet += (char) LoRa.read();
}
Serial.println(packet);
delay(5);
} ```
``` // writer
boolean sendPacket (String packet) {
Serial.println("Send begin");
LoRa.beginPacket(false); // true: optional implicit mode (--> Set everything on both sides?!)
LoRa.setTxPower(14,RF_PACONFIG_PASELECT_PABOOST);
LoRa.print(packet); // also LoRa.write(byte(, length));
LoRa.endPacket(false); // true: async mode: doas not wair until transmission is completed
delay(250);
// put the radio into receive mode
LoRa.receive(); // set redio back in receive mode
delay(750);
Serial.println("Send end");
return true; // will be changed
}
void loop(){
while(Serial.available() > 0 ){
delay(2); //delay to allow byte to arrive in input buffer
String text = Serial.readString();
digitalWrite(LED, HIGH); // turn the LED on (HIGH is the voltage level)
boolean packetSent = false;
while (!packetSent) {
packetSent = sendPacket(text);
if (packetSent) {
Serial.print("Packet has been sent: ");
Serial.println(text);
} else {
Serial.print("Retry sending packet: ");
Serial.println(text);
}
}
digitalWrite(LED, LOW); // turn the LED off (HIGH is the voltage level)
}
} ```