Problem with reading a complete string on a serial port

Problem with reading a complete string on a serial port

Dear all.

I have a problem with a reading a string on a serial port.
I can send string, send character, read character but I can't solve reading of complete string.
I have two similar functions in a unit for reading from a serial port:

function TPLC_Comm.ReadStr(var S: String) : Boolean;
var
TempArray : array[1..255] of Byte;
Count, RX_Count : DWord;
begin
S := '';
ReadFile(Com, TempArray, 255, RX_Count, nil);
for Count := 1 to RX_Count do S := S + Chr(TempArray[Count]);
Form_Glavna.Edit_ToPrint.Text := S;
if length(S) > 3 then Result := True;
end;

function TPLC_Comm.ReadChr(var C: Char) : Boolean;
var RX_Count: DWord;
begin
ReadFile(Com, C, 4, RX_Count, nil);
Result := (RX_Count=1);
end;

So in Execute for reading a character I do something like this:
Repeat
If ReadChr(Ch) then
Begin
IF (Ch=$#06) then ...( do something);
If (Ch=$#15) then ...(do something2);
End;
Until (Ch in [$#06, $#15];

This working great. But if I use a same method but with function of reading a string like:
Repeat
If ReadStr(S) then
Begin
If length(S) > 3 then ... (do something 3);
End;
Until length(S) > 3;

It happens that from time to time I miss a complete string and usually I do not receive a first character in a string.
Does anybody have an idea how to solve this?

Thx
Tomislav