I know exactly what you mean! Even a couple of my family refuse to accept that PCS Is more than a headache. My brother has relentlessly teased me for my 'pathetic behaviour' and 'constantly being ill' and while he doesn't mean any harm, he just doesn't understand, when I told him it's almost similar to a hangover but so much worse and is never ending. He was still confused by it. My grandmother as been by first the worst, constantly asking if I've drunk enough water or telling everyone that I get headaches because I don't eat enough. It's infuriating!
As for my friends, most of them accept I got a concussion, all of them were there when I got it, but when I struggle to get up and find it painful to move they laugh and ask if I'm suffering lazyitis!
No one seems to understand, no matter how many times I try to explain... Nothing... I completely understand how you feel. I think some people are naturally more understanding than others, and other people are concerned and try to deal with in their own way. In the case of my grandmother, trying to find a quick fix cure or not knowing what a syndrome is.