Why can't Googlebot see half of the site?

Question

Google

In the search console, when crawling as a Google bot, half of the site is not visible, what could be the problem?

Answer 1

In anything
Everything is exactly the same as in the cases of
"My finger hurts, what's the matter?"
"My car won't start, what's wrong?"
"Outside the window is white. What is it?"

Answer 2

• Too many links on one page.
  There may be many other reasons, depending on the specific site.