Had the exact same problem about six months ago when I was trying to get BigCommerce orders to flow into IOM. Followed the Microsoft docs step by step and everything looked connected but zero orders were coming through on the Dynamics side.
What fixed it for me was realizing the orchestration flow wasn't actually activated — it showed as set up but wasn't running. You need to go into Order Management > Orchestration flows and manually flip it on for your BigCommerce provider. Also check your order field mappings because BigCommerce sends order statuses differently than what IOM expects by default, and if there's a mismatch the sync just silently fails with no error.
I'd also look at Data Management > Integration logs to see if anything is getting stuck there. In my case there were failed records sitting in queue that I didn't even know about until I dug in.
I ended up hiring a team that specializes in BigCommerce development to sort out the whole integration properly because the official docs really don't cover the edge cases. Once they fixed the mapping and orchestration config everything started syncing within minutes. Saved me weeks of guessing.