SD opens ONLY some files

Hi, I don't know if this is already solved in another post, but I didn't find it.

The problem is that my Micro SD module iniciates and reads some ".txt" files which are called "mapa___"
but then when I try to open another file with the name structure "mapasup___" it doesn't open.
The "___" is the place reserved for a number.

#include <SD.h>

File myFile;

String copy;
const String txt_s=".txt";
const String mapa_s="mapa";
const String mapasup_s="mapasup";
int n_mapa=22;
float mapa[384];
float mapasup[384];
int i;

void setup(){
  Serial.begin(9600);
  if(!SD.begin(7)){
    while(1);
  }
  pintar_mapa();
}

void loop(){
}

void pintar_mapa(){
  //COPY MAP
  myFile=SD.open(mapa_s+n_mapa+txt_s);
  Serial.println(myFile);
  if(myFile){
      //SOME MORE CODE
      myFile.close();
  }

  myFile=SD.open(mapasup_s+n_mapa+txt_s);
  Serial.println(myFile);
  if(myFile){
      //THE SAME MORE CODE
      myFile.close();
  }
}

So if I open the Serial the answer is myFile=1 for the first part and myFile=0 for the second try.
If you need more information ask me pls. Thank you for reading!

From the SD library's reference page:

It uses short 8.3 names for files.

This means the filename before the extension can be a maximum of 8 characters long, and the extension can be a maximum of 3 characters long. So if the number is more than 1 digit, you have exceeded the 8 character maximum.

Wow, that was easier than I thought. I changed the files name to "m___" and "ms___" and everything is working fine.
Thanks a lot!!

You're welcome. I'm glad to hear it's working now. Enjoy!
Per