If you suffer from sinus inflammation, either from an temporary infection or a chronic condition, they do not open.
If you have problems in your ear, such as a perforated eardrum or an inflammation; you may still be able to equalise pressure, but it can be painful to do so.
Other genetic or congenital conditions exist that can prevent or hinder pressure equalisation.
Ah, I see... Hmm, I imagine air travel would be quite painful in that case. Why doesn't the cabin pressure perfectly equal ground pressure? Doesn't it make sense that an airtight cabin will keep the pressure it's sealed at?
It's not that easy. The pressure difference between the inside and outside puts a great amount of strain on the fuselage. I think what they do is just reduce the rate at which the pressure changes.
The pressure difference between the interior and exterior of the aircraft causes considerable stress on the aluminum. One of the advantages with composite fuselages is their greater durability. This will allow for pressurization to an equivalent 6000ft altitude, as opposed to the current standard of 8000ft. This will help with numerous things, including but not limited to: air quality, ear popping, humidity, etc.
Yep, that also opens the tubes. It doesn't help when the pressure differential is too high, though, as that takes concentrated effort. I can voluntarily do it and even I have to try hard at getting them to open when the pressure differential is high, which is why I take care to do it regularly while ascending.
Two things work. Nose spray (unblocks nose and the whole canals). And sinutab an hour before landing...
I'm no reseller of those I'm just sharing for the ones who've been in horrendous pain like me...