I was having this same issue. What's working for me right now is to remove the products from Facebook sales channel and then re-enable them. It seems to jog the system and causes them to be resubmitted successfully. It takes a few minutes for them to show up in Facebook Commerce Manager, and then some time for FB to "review" them. It's worked so far! I used Bulk Edit.
I have worked it out! You need to go onto your Instagram Profile, select 'View Shop', then go to the top right three lines, select 'Manage products', and then 'Add Product'. You will then be given the option of what products you wish to add. The products that you have added to Facebook / Instagram shopping manually, via Shopify, should all then be showing on this option.
This took me ages to work out, I was focusing on Shopify rather than Instagram. Really hope this helps people.