I was stumped for a little bit why my products was not showing up in my collection. First I made sure all my products had the same tags as I have in my collection. I also made sure each product was active, all have the same sale channels. After doing these things my products still was not showing in my collection. So when I went to my shop and clicked on my collection name nothing still was there. So I said I know what I am going to go in navigation and delete my coffee mugs collection and re add it to my navigation. So when I added it back I went to link and found my collection and then it pop up the collection name and it automatically fill in the name of my collection. I didn’t used any tags for my collection in navigation. I save it and then went back to my shop and clicked on coffee mug collection and there was all my products for that collection now showing like they should. I hope I explained this right and it help you.