Once setup, your connector sits there in the background synchronising Invoices, Accounts and Products. Sometimes though something just won't sync and there are a number of reasons why this may happen. In order of likelihood these are:
3.5.0 Invoice created before you installed the Stellarise Connector managed solution
3.5.0 Date Delivered on Shipping Dates in does not exist or is in the future
3.5.0 No Stellarise Connector selected
3.5.0 The Account and Tax Codes are not synchronised
3.5.0 You are trying to Synchronise an invoice but the Invoice ID already exists in Xero