It could also be the new soil as well. Morning Glories prefer poor soil that is slightly acidic. I live in the Southeast in zone 7a. I have never had a problem with letting many of the common morning glories self seed (except for the tropical varieties like Moonflower, which tend to rot). When they find a place they like, the common, tievine, small, and wild bindweed morning glories grow like wildfire here and take over yards. That is unless the birds eat the seeds while you're not looking, in which case you will find a seed pod with no seeds, half a seed pod with no seeds or no seed pod at all. It may be too cold for them to self seed up there, but I say give it a try... Or, collect the seeds, store in a dry location, then do about 60 days of cold moist stratification, and replant when you know temperatures won't get lower than 45 degrees. Start them in a pot indoors, and then set them outside in the late Spring or Summer. If you're buying new soil, I would recommend using a mix of half topsoil and have garden soil It could also be the new soil as well. Morning Glories prefer poor soil that is well-draining and slightly acidic. I live in the Southeast in zone 7a. They love growing on rock piles, and I have even seen them in solid red clay dirt. I have never had a problem with letting many of the common morning glories self seed (except for the tropical varieties like Moonflower, which don't like how I plant them at all. Lol).
When they find a place they like, the common, tievine, dwarf, and wild bindweed morning glories grow like wildfire here and take over yards. That is unless the birds eat the seeds while you're not looking, in which case you will find a seed pod with no seeds, half a seed pod with no seeds or no seed pod at all. It may be too cold for them to self seed up there, but I say give it a try... Or, collect the seeds once the seed pod is brown and dried, store in a dark and dry location, then do about 60 days of cold moist stratification, and replant when you know temperatures won't get lower than 45 degrees. Start them in a pot indoors, and then set them outside in the late Spring or Summer.
Again, the tropical morning glories (Moonflower and Elephant Creeper) don't like me, so this is just for the others... If you're buying new soil, I would recommend using a mix of 1 part topsoil to 1 part basic garden soil with perlite added (in whatever quantity is recommended on the package). Google how to make your soil slightly acidic, and do this as well. I believe all it takes is a bit of vinegar or unused coffee grounds, but it can be done relatively inexpensively. Hope this helps!
Good luck!