Google analytics not showing all visitors?!

Hi everyone,

I run a online marketing & services company. For a client I post on Facebook ads + google short urls to a landing page. However, it seems that Google analytics only tracks half the people who visit the site.
I've experienced this before on this site, I had over 800 hits on the short url but analytics showed only 200 visits.

I've read on internet that Google only tracks when cookies are available. But my audience in this case is 80% over the age of 40. The half or majority of this group don't block all cookies, do they?

Image:
photo uploading

Anyone knows what causes the problem here or knows a fix?

Thx

i think it's about how long they stay in your website

vpn's - extensions - add ons
most of them block tracking codes - javascirpt codes - cookies - etc
and many many more ways.