Arduino not saving to SD correctly - arduino

Question

I've built simple project with my daughter that takes two temp readings and writes the to a file on a Micro SD card.
The code is fairly simple (if not a bit messy) and is creating the file, but only ever seems to write the same row of data twice (from the setup()) and then never writes again, although doesn't seem to produce an error.
The code is:
#include <Wire.h> //Required for Realtime Clock
#include "RTClib.h" //Required for Realtime Clock
#include <SPI.h> //Required for SD card reader
#include <SD.h> //Required for SD card reader
#include <dht11.h> //Required for Temp/humidity reader
#define DHTPIN 4 //Pin for temp sensor
#define LEDPIN 7 //Pin for led (status)
#define DHTTYPE DHT11 //define which temp sensor we are using (DHT11)
dht11 DHT11; //Create instance of DHT11 class to read sensor values
File myFile;
String filename;
uint32_t start_min; //Store time the current loop started so we can work out how long we've been running for
RTC_DS3231 RTC; //Create instance of real Time clock class
void setup() {
Serial.begin(9600); //start serial monitor for output
Wire.begin();
//begin the rtc, let us know if it fails
if (! RTC.begin()) {
Serial.println("Couldn't find RTC");
abort();
}
Serial.print("Initializing SD card...");
if (!SD.begin(10)) {
Serial.println("initialization failed!");
while (1);
}
Serial.println("initialization done.");
//set RTC time and date to the time the app was compiled
RTC.adjust(DateTime(__DATE__, __TIME__));
//Get the current RTC time and sore it as a unix time
DateTime now = RTC.now();
start_min = now.unixtime();
//Setup Filename
String filename = String(start_min);
int start = filename.length() - 8;
int end = filename.length();
filename = filename.substring(2,10) + ".csv";
Serial.println(filename);
//Write Header row
myFile = SD.open(filename, FILE_WRITE);
if(myFile) {
myFile.println("timestamp|humidity|temperature1|temperature2");
myFile.flush();
myFile.close();
delay(1000);
}
else {
Serial.println("Error opening file");
}
}
void loop() {
//Get current RTC date and time
DateTime now = RTC.now();
//Convert to unix time
uint32_t _now = now.unixtime();
// work out if x mins have passed
if((_now - start_min) > (4*60)) {
//Turn LED on
digitalWrite(LEDPIN,HIGH);
Serial.print("Looped - ");
Serial.print(now.hour());
Serial.print(":");
Serial.println(now.minute());
int chk = DHT11.read(DHTPIN);
myFile = SD.open(filename, FILE_WRITE);
if(myFile) {
String output = String(now.getTimeStr());
output += "|";
output += String((float)DHT11.humidity,2);
output += "|";
output += String((float)DHT11.temperature, 2);
output += "|";
output += String(RTC.getTemperature());
Serial.println(output);
myFile.println(output);
myFile.close();
Serial.println("Write complete");
}
else {
Serial.println("Error opening file");
}
//reset start time
start_min = now.unixtime();
}
delay(5000);
//Turn LED off
digitalWrite(LEDPIN,0);
//wait 90 secs
delay(5000);
}
I'm using an Arduino Uno, a DHT11 for temp/humidity and a DS3231 RTC (also grabbing temp there to).
Once compiled and uploaded. this is the output:
Sketch uses 16798 bytes (52%) of program storage space. Maximum is 32256 bytes.
Global variables use 1290 bytes (62%) of dynamic memory, leaving 758 bytes for local variables. Maximum is 2048 bytes.
any help appreciate