Hi & sorry for the delay. Besides the things already suggested in this discussion, the other fix I discovered was in the product variants. If you name your own variant, that causes problems. It’s best to use the given variants. I found ‘Style’ covers most of my needs.
Besides this, I can suggest not adding tags to the box that pops up regarding collections. Sorry, I can’t remember when & where this happens but it’s best not to add tags there (though of course do add them within the editing of collections otherwise no products will show up). Hope this helps. If not, do contact Shopify support. They’re very good.