(1) The body is not loaded properly
When the body is loaded properly, the back and arms are all in contact with the seat and the feet are on the floor.
(2) No postural support
If a pressure cushion has been stacked on top of an existing standard cushion, or placed on a chair that is too wide, it can reduce the effectiveness of the arms, lateral supports and headrests in holding the person in a good posture.
(3) The chair is already unsuitable
Adding a pressure cushion to a chair that is already unsuitable for the purpose will not solve the problem fully. If the dimensions of the chair are too big and it does not provide postural support, simply adding a pressure cushion will not be an effective solution for the person to benefit from the pressure cushion. https://www.evidence.nhs.uk/search?ps=100&q=pressure+relief+cushions