CoderByte says all of my testcases are wrong?

Question

I tried coderbyte and looks something is wrong with it. The first challenge is simply to reverse a string. I did this in java:

import java.util.*; 
import java.io.*;

class Main {

  public static String FirstReverse(String str) {
    char[] chars = str.toCharArray();
    String ret = "";
    for (int i = chars.length - 1; i >= 0; i--) {
      ret += chars[i];
    }
    return ret;
  }

  public static void main (String[] args) {  
    // keep this function call here     
    Scanner s = new Scanner(System.in);
    System.out.print(FirstReverse(s.nextLine())); 
  }

}

It said, that three test cases had wrong output and there was the correct output. I tried running the code with the specified cases and it printed the same string as correct output for that case. So I tried resubmitting it and it said that only one test case was correct and all other had wrong out put. So I said OK and rewrote my code this way:

import java.util.*; 
import java.io.*;

class Main {

  public static String FirstReverse(String str) {
    return new StringBuilder(str).reverse().toString();
  }

  public static void main (String[] args) {  
    // keep this function call here     
    Scanner s = new Scanner(System.in);
    System.out.print(FirstReverse(s.nextLine())); 
  }

}

Unfortunately it still says only one test case was successful... Any ideas what happened? Thanks