I get this question a lot, “How come when we change our site logo for the site collection that it reverts to the default logo on some of our pages?” Well 9 times out of 10, those pages you are referring to are Web Part Pages. Those types of pages have their own logo, and if someone knows the official reason why to this, please say so in the comments. So, in order to keep our site uniform with the logo, then we need to add it here as well:
Go to the page that has the offending default logo
On the Page ribbon tab, select Title Bar Properties
Scroll to the right and on the Web Part Page Title Bar properties paste the URL to your desired site logo
Click OK and you’re done!