I thought this was just happening to new collections, but it turns out it's not just new ones, because I made a blank collection two weeks ago, and when I went to add products today from my store back end, that collection did not come up in the collection list. I had to go to the collection management and add a few products, and THEN I could add the rest from my store back end. So i think there's some sort of glitch that doesn't let us add products to a collection unless there's already some products in that collection.
Here's a link to the collection I was working with- not sure if the glitch is only in this collection or not. https://www.zazzle.com/collections/119670473013551526
