Existing file on SD-card not recognized

A trough existing file on SD-card is not recognized

In the setup part, the following code is written

// see if the card is present and can be initialized:
  if (!SD.begin(SD_Select)) {
    Serial.println("Card failed, or not present, in setup");
    // don't do anything more:
    //return;
    }
    else
    {Serial.println("card initialized.");}

In another subroutine (Store2SD) I can assemble a variable filename (f.i. CVjjmmdd.csv), but in undermentioned routine not! The existing file Offsets.csv will not be recognized. (there are no open files on the SD-card). I dont get error messages from the IDE.

void Equalizing_sensors()
  {    
    //Declare local variables
    File myFile;
    String strDatastring = "";
  
    //Call temperature values
    temperature_1 = sensors.getTempCByIndex(0);
    temperature_2 = sensors.getTempCByIndex(1);
    temperature_3 = sensors.getTempCByIndex(2);
    temperature_4 = sensors.getTempCByIndex(3);
  
    offset_1 = ((temperature_1 + temperature_2 + temperature_3 + temperature_4)/4 - temperature_1); 
    offset_2 = ((temperature_1 + temperature_2 + temperature_3 + temperature_4)/4 - temperature_2); 
    offset_3 = ((temperature_1 + temperature_2 + temperature_3 + temperature_4)/4 - temperature_3); 
    offset_4 = ((temperature_1 + temperature_2 + temperature_3 + temperature_4)/4 - temperature_4); 
    Serial.print (offset_1); tab(); Serial.print (offset_2); tab(); Serial.print (offset_3); tab(); Serial.println (offset_4);
  
    EEPROM.writeFloat(100, offset_1);
    EEPROM.writeFloat(104, offset_2);
    EEPROM.writeFloat(108, offset_3);
    EEPROM.writeFloat(112, offset_4);
       
    //If file exists open it, or make a new file 
    //Note that only one file can be open at a time, so you have to close this one before opening another.
    if (SD.exists("Offsets.csv"))
        {Serial.println("File bestaat");
        myFile = SD.open("Offsets.csv", FILE_WRITE);
        }
        else 
        {Serial.println("File bestond niet");
        myFile = SD.open("Offsets.csv", FILE_WRITE);
        strDatastring = String ("Datum/tijd") + ";" + "T1" + ";" + "T2" + ";" +  "T3" + ";" + "T4";  //Write a header
        myFile.println(strDatastring);
        }   

    // if the file is available, write to it:
    if (myFile) 
       {Serial.println ("Gekomen tot achter myFile");
       strDatastring = strDate + " " + strTime + ";"; 
       dtostrf(temperature_1, 4,1,outstr);
       strDatastring += outstr ;
       strDatastring += ";";
       dtostrf(temperature_2, 4,1,outstr);
       strDatastring += outstr ;
       strDatastring += ";";
       dtostrf(temperature_3, 4,1,outstr);
       strDatastring += outstr;
       strDatastring += ";";
       dtostrf(temperature_4, 4,1,outstr);
       strDatastring += outstr;
       myFile.println(strDatastring);
       myFile.close();
       }
      else 
      {
      // if the file isn't open, pop up an error:
      Serial.println(String("error opening") + myFile);  
      }   //End of if

 //Put a message on LCD
    strLine2 = "De kalibratiegegevens zijn";
    strLine3 = "   opgeslagen";
    strLine4 = ""; 
    blnEqualizing_done = true;
    } //End of equalizing_sensors

Has anybody a suggestion?

Can you recognize Offsets.csv in your Store2SD function ?