Is it length of column name?
Try to keep it <=256
<Moderator Edit - deleted link spam>
I was talking about the column value inside the column and not the column name.
The length of the column value is 67 and just appears 28 characters.
The value of this column is getting by a function.
Does it limit the number of characters?
I have call the function inside a cast to varchar and appeared all the characters