ResMed Mirage Activa LT User Manual page 12

Possible reason
Mask is incorrectly assembled.
Cushion may be dirty.
Cannot insert forehead support into mask frame
When assembling the mask, you
have inserted the dial into the frame
before the forehead support.
Solution
Reassemble the mask according to the
instructions in "Reassembly" .
Clean cushion according to instructions in
"Cleaning the Mask at Home" .
Push dial base onto a hard surface until the dial
comes out of the frame.
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