When the Lead get Converted and it was synced in Pardot, But after that if was stop syncing then related List

My issue is that, we have a converted lead that was already synced in the pardo, After the we have created a custom Object, related to the lead, and when we put the Converted Lead Id in out Newly custom Object then, that record was not appearing in the Pardot?
Any Guess why this is happening?