Sagging rattan or cane seats of conservatory furniture can be restored
Unfortunately over time the seats of conservatory chairs constructed from natural rattan or cane can sag.
If the sagging is only slight just sponge down the seat area with tepid water and leave to dry for at least 24 hours before using. This should cause the fibres to contract and shrink back into shape. Spray occasionally with a fine plant mister to ensure the fibres stay taut. Making sure you always use a cushion will also help keep the seats taut as they help distribute the weight. If the chairs are designed to be used without cushions then they will have extra support underneath the seat to prevent sagging.
If the sagging is excessive then the only remedy may be to either pile on the cushions or replace the chairs.