Welcome to our dropshipping community here! So excited to watch you grow from newbie to successful dropshipper. You got this!
This situation is certainly a little odd! My only idea of why one collection out of three is not being updated by the Global Pricing Rules is because it might not be properly sync'd to Oberlo. If it's not sync'd, every time you update the pricing rules, it's only registering two instead of three.
Could you please try to follow these steps to sync the products in the collection to Oberlo again? Then, try to "apply all" with the Global Pricing Rules.
If that still doesn't work, please shoot a message to our support team at email@example.com. They can log in to your Oberlo app and actually test it from their end. If something is going on, they can escalate it to our developers to take a look. One way or another, we are going to figure out what is going on here!