Trying to Place Ads Across your multi-site? There's a trick that you can use.

Almost everyone didn’t understand, there’s an easier way to run ads across your entire multi-site. Within this post, I’ll show you how to implement ad banners on your multi-site, along with some potential problems that you may encounter:
On single-sites, you can use a plugin, Header & Footer to easily place ads either above or below posts, and pages. Each post or page has these checkboxes to enable/disable top/bottom injection. You can do this to each site via a multi-site, but this plugin doesn’t had any multi-site functionality just yet.
The another way you can implement banners on your site is by editing your templates directly. This is useful if you are hosting your blogging platform for some users who wanted to create a free blog on your multi-site. Whenever you placed a banner at the search results page, index page, header, footer, sidebar, etc. To do this; follow these steps:

1. Go to your website dashboard, after wp-admin, type network at the address bar. And go to Editor.
2. Go to any of your desired themes, and edit your template. Place the ad code inside a div container (wrapped in div tags). Always place it after the title element. For index pages, place it above the php script,–where the index is displayed. Be careful NOT to mix up the code,–otherwise it will not work.
3. Save your template. Open your browser, and view your site. Go to your post page to confirm your ad banner is showing, if it’s showing. That means your banner is working properly. Now your site will show banners without any effort. You can repeat these steps on each theme.

Overtime, when your themes get updated, you might lose your banners, you should always reinsert banners on the regular basis.
Why this trick?
Useful for hosting a free blogging platform. Most free blogging platform use ads to keep their blogging platforms free of paywalls,–which is necessary to keep your revenue stream active. Like wordpress.com; they use their ad system to keep their blog/service running.
If the ads won’t appear, or it will NOT appear on your multi-site; it could be a problem with an ad network, your monetization account via third party ad platform what you’re using. You need to check with your ad provider if they can fix your problem.
Note:
If your site is susceptible to heavy UGC (user-generated content); your banners might overlap some content. BR tags are needed to make space for your banners that are expandable.
Common problems with implementing banners on your site is the following.
Your website themes might have problems with the ad code somewhere in the template file.
On the front-end, your banner might caused your webpage to slow down.
Or your banners not displaying at all.
Check with your ad network provider if they had any features that prevents your ads from being rigged with UGC.
Always place a label ‘Advertisement to clearly divide site and ad content.
Alternatively, you can place your banners at the footer of your site by choosing a footer template file.
If you wanted to see an example; you can visit my site at [link moderated] to see for yourself.