Related products may appear unrelated in WooCommerce because the default algorithm uses categories and tags to find similar items, but you don’t have direct control over the selection unless you use a plugin or custom code. Inconsistently organized categories and tags, or certain theme limitations, can also lead to unexpected suggestions.
Common causes and solutions

Broad categories or tags:
The most frequent reason for seemingly unrelated products is that they are all in the same, but very broad, category or share a tag that isn't specific enough. 

Solution: Make your categories and tags more specific. For example, use "Morning Video" instead of "Morning" as a tag, as explained on WordPress.org. 

Default algorithm randomness:
WooCommerce selects products randomly from the pool of eligible items (those in the same category or with the same tags).

Solution: To gain more control, use a plugin that allows you to manually choose related products, or use custom code to create a more specific rule, according to Omnisend and WordPress.org. 

Theme limitations:
Your theme may influence the related products display or have its own settings that affect which products are shown.

Solution: Check your theme's documentation or contact the theme developers to see if they have settings to adjust how related products are displayed, notes Themeco and WordPress.org. 

Caches:
Sometimes, your browser’s cache or a site-wide caching plugin can prevent you from seeing the most up-to-date related products.

Solution: Clear your browser cache and purge your site's cache, suggests Themeco. 

How to manually influence related products

Tags:
If you want to show products from different categories together, use tags to link them. You can then ensure these products are more closely related. 

Up-sells and cross-sells:
Use up-sells to recommend a better or different product, and cross-sells to suggest complementary products in the cart. These are separate from the default related products and can be set manually.
Plugins:
Consider using a plugin for more control. For instance, some plugins allow you to explicitly choose which products are related or link products by tag instead of category, explains a WordPress.org forum post.

For a comprehensive guide on setting up related products to boost sales, you can read this article from FunnelKit. If you are having trouble with the appearance or position of related products, you can find more information in this WordPress.org forum thread. For a guide on using shortcodes to display related products, visit this WooCommerce documentation page.