Fitting through the opening does not prove that a duvet suits the machine or programme. Its care label, dry size, filling and need for movement all matter.
Understanding this situation
A bulky duvet can become a dense wet mass even when its dry weight appears acceptable. The machine's general cotton capacity is not automatically the permitted duvet capacity, and seams or fillings may impose separate care restrictions.
A useful next step
Check both the bedding label and the model's bulky-item guidance. Use another appropriate cleaning method if suitability cannot be established. Do not compress the duvet into the drum or force the door closed to make it fit.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.