Hairnets are a fickle creature,
First i would understand the issue of hair, lice, dead skin, ear wax, ear hair, and oils is mitigated from products if worn correctly as designed, this includes bald men or men who shave their head.
Second i would ask what is the impact on your process and products if hairnets were not worn correctly, i.e if said contaminants were to enter the process or become embedded in your products. If i found bits of hair lice embedded in my packaging i would use a new supplier, and damage to name brand could suffer.
If you find that A the risk and severity of contamination being present if not worn exists and is unacceptable to you then i would say 1) wear them and 2) wear them properly as best practice in food manufacturing or manufacturing of food grade materials would require it over the ears.
And you didnt ask about Snoods which to me is the same deal. over the mouth under the nose if tight but generally over the nose is common.
There to me should be no question about staff not wanting to do it, because end of the day, if they dont understand why they should wear them properly then they should not be working for you. Those who take pride in their work with a sense of pride should understand the need to wear them and not argue the point.
Make a company policy, show a poster of what good looks like, and set that as your site wide standard!.
But you know you will always get those smart people who question you why some places mac D, Greeeegs and the like who have staff who some wear just hats, some wear just hairnets and some who wear combinations of both. but then LOL look at the makeup they wear and the fake nails, lashes etc.
On the topic of hairnets i have a similar issue which i shall post in my own thread :D