GSM sim800L receive sms problem

Hi Everyone i need your help. i am working on GSM and have to receive SMS from Sender. but in serial monitor i got some problem with receive serial.
  //Gsm.println("AT+CMGD=\"DEL ALL\"");
  Gsm.println("AT+CNMI=2,2,0,0,0");  //procedure, how to receive messages from the network
void loop(){
      msgByte =;
      msgData += msgByte;
    msgData = "";

Ready to received Commands...

HҨ⸮$⸮%MM12p1f⸮⸮⸮⸮⸮⸮	⸮)⸮⸮%&⸮
Message: Hj⸮⸮ª⸮⸮⸮96x874E
Here the problem 
i send E8347x937x3488x96x874E
Thanks in Advance!
strong text

What kind of power supply do you use ?

I used LM2596 DC-DC buck converter. I think it worked find, bcuz LED blink in 3s. And sometime i can receive sms with missing characters:
Ex: i send : H38374x8787x9786x98E
Result : H38374x8787x
I changed the buffer in software serial form 64 to 512 or 128 or 256
but still missing and some time random Message like [Hj⸮⸮ª⸮⸮⸮96x874E]

I is use A6 chip And the strange char ( ⸮) appear when there is an undervoltage situation, try this:
If when send a sms is ok but when receive the ⸮ appear is probably a power supply problem. Ps in my runtime i cut the finction wich read gsm serial And print on usb serial And it work properly ( sorry for my bad english lenguage).

Thanks You So Much!:slightly_smiling_face:

Did @JKwatt suggestion work?

Did it work?
I have a similar problem.
when i send an SMS to to SIM inserted in the GSM A6 module, i only get the phone number, date and time, but not the message itself