A flat duvet has a cause. Sometimes the material itself is tired. Sometimes the down is simply dirty from use. The distinction matters.
Cause one: quality
Down of lesser quality can lose its volume when feather quills break or the filling becomes too brittle. Recovery is then limited. There was simply less resilience to return to.
Cause two: dirt
Sweat leaves salts and oils on the clusters. Those clusters then open less well. The down lies flat, not because it is finished, but because it is restrained.
What this means for you
Good down often opens up again after professional washing. The natural lightness returns. With lower quality, refilling sometimes helps, but the original resilience does not always come back.
Four habits
Regular airing, timely washing, light shaking and weekly changes keep a duvet longer in good shape. Not a heavy daily task, but it works.
First look, then decide
A flat duvet is not necessarily lost. Sometimes the filling is mostly affected by moisture, salts or dirt. Then professional cleaning can restore much. Sometimes the quality of the filling itself is the problem. A craftsman sees that after inspection. Important: we avoid dry cleaning, the chemical cleaning weakens the down clusters.
What you can notice yourself
Look at the colour of the casing, the volume of the duvet and the distribution. Does the filling stay in clumps together? Does the duvet feel colder or heavier than before? Such signals say it is time to have someone check what is needed.