Infrequent flyer
My guess is that masks failing to drop might be related to no oxygen pressure - or low pressure further away from the supply
Swedish Steve
introduce oxygen at about 16psi into the same main. This higher pressure opens the PSU doors and allows the masks to drop.
I will try and explain more slowly. Oxygen pressure opens the latches and the panels in the PSU fall open. The masks are now sitting on nothing and will fall when gravity lets them.
The masks and the coiled up plastic tube are a snug fit in the boxes. Occasionally they will not drop by themselves and need a nudge. It really depends on how well they are packed. I personally find it quite difficult to roll up 4 masks and stow them in a box in situ, but it beats taking the boxes out on a transit!