Answer: Sword ferns aren't subject to any particular disease problems so I suspect there's something amiss with your soil. If it drains poorly, the root systems of your ferns may be adversely affected. Root rot would cause the ferns to flop over as you describe. If you know the soil drains well, then some ground-dwelling critter may be compromising the root systems of your ferns by gnawing on them or tunneling through them. Dig one of the affected ferns up and check the root system. You should be able to tell if the roots have been chewed through or have died off from excess moisture. Once you uncover the problem, you can correct it by amending your soil with organic matter to help it drain quickly, or caging the root systems of your ferns to keep moles from gnawing through them.
Best wishes with your garden!
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