If you can't figure out why an account hasn't synced from Recurly -> Salesforce, check your Deleted Accounts page in your Salesforce org. Accounts listed here will never sync unless removed. To remove, click on the deleted account and then click the downward arrow -> Select Delete:
Once you do that, the account will re-sync over to Salesforce the next time it is updated. If you need it synced immediately, you will need to go to Recurly Admin -> Sync Settings -> Edit -> Set the date to a time in the past and then Save.