I added products to my store over the last 3 weeks and so far none of them have showed up in my pinterest products board. This used to work well in the past although it took a day or 2 to sync up. I disconnected and reconnected my pinterest account and still no luck. The products are available on my store and I checked the box for the pinterest sales channel.
I had this same problem and I just found the solution.
So on your products page in Shopify, filter the availability to "Unavailable on Pinterest"
And then on each product, click Manage under "Sales Channels and Apps" on the right-hand side right under Product status. Click select-all, done, and save. That product will now show up on Pinterest
The only annoying part is that you have to do that for each product which will take some time. Hope this helps!