A few possibilities come to mind when products in a collection don't appear on the website. First, the status of your products could be set to Draft instead of Active. Products need to have an active status for them to be published on the website. Another possibility is that your products may not have been set to be available in the Online Store sales channel. They need to be available in the channels you wish for them to appear. Lastly, you may have filter tags when you set up the collection as a menu item. The website may not find any products that have the tags you've set and when that's the case, no products appear.
I've shared sets of steps below that you can follow to make adjustments accordingly for each situation. Feel free to go through them to troubleshoot the issue.
Draft status instead of Active
Products aren't available in the Online Store
Irrelevant filter tags are present in the menu item
If none of these make the products appear in the collection, we’ll need to access your account in order to investigate and help you resolve the concern. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback. Please visit https://bit.ly/3cJkx8V and log in to your account to create a support request.
While we're on the topic of collections, make sure you optimize them so they have the ability to boost your sales. It may not seem like it but the way you set up collections can significantly help your business in the long run. When collections are easily accessible and have the elements that can push customers to purchase your products, they can raise the number of sales in your store. To know how you can configure your collections to your advantage, feel free to read this article that shares best practices on setting up collections to drive traffic and boost your store's credibility.