I think that's the most likely thing. Does it sag at all when it's extended with washing on it?
You can pull it pretty taught and it doesn't slip. I actually use a prop in the middle because it raises the line higher and gets the clothes, drier quicker but that's by no means essential.
@Quincy started
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I think that's the most likely thing. Does it sag at all when it's extended with washing on it?