Did you know that there is a limit of 150 wiki pages that can be displayed in the Insert Content into the Page Menu?
That means if you have more than 150 wiki pages in a course, the page you are trying to link to may not show up in the Insert Content menu and you will have to grab the URL manually.
I've entered a feature request to increase that limit so all pages in a course will show up in that menu when editing a page.
Please check out this feature request. I think it is something Canvas should implement.