After using text file through script does not show some elements in apk

Hi, I've used some text file through script then i build apk. but after using text files some elements (including unity4.6UI) can't be shown in mobile. everything was fine before using text file. please help. P.S.: i used different class for reading text file.i just access letter list from another script. sorry for my bad English.

here's the code

public class LetterPick : MonoBehaviour { public static LetterPick instance;

 internal List<string> letter = new List<string>();

 internal StreamReader sr = new StreamReader(@"Letters\letter.txt");

 public void Awake ()
 {
     instance = this;
 }

 public void Start ()
 {
     SetLetters ();
 }

 public void SetLetters ()
 {
     string s;
     while(!sr.EndOfStream)
     {
         s = sr.ReadLine( );
         letter.Add(s.ToUpper());
     }
     sr.Close( );
 }

}