ResMed Ultra Mirage™ II Air Vent Cover

I don't understand why these are not permanently attached to the vent. They pop off in the night and are difficult to find and impossible to sleep without. The ones you sent were fine except that they don't fit tightly and again pop off constantly. However, that being said, I am glad to find a place to buy them since the company we use for supplies can't seem to understand what I am asking about.