How old is the string, When I switched to a compund a few years back, my xbow sat. SOn decided to take up hunting so let him use it, we found the same thing, and the string was lose , so put new string on and then did like others had said start at 10 yards, and work back .
With it being an older style you have to make sure you pull back as straight as possible, if you are 1/4" in drawing back, to one side that can make a huge difference at 20 yards, and if string is lose makes it even worse,
Just my limited experience.