How can I see which transactions have not synced to Raiser's Edge via the connector and why they haven't synced?
This can be done through the Unsynched Transactions Report, which is run just like any other data sync.
Go to Data Management > Import/Export
Go to the "All Data Sync Operations" tab
Search for the operation called "Convio Connector for RE Unsynced Transactions" and click "Run"
This will generate a file of all of the transactions that have not synced and why. If a transaction says "This transaction now qualifies for the next data sync run", it means that the transaction has not yet synched but is now eligible to sync, so these can be ignored. The rest of the errors will generally point to missing cross references. Whenever you see an error that indicates an ID is missing on the constituent record, that generally indicates the the constituent is new and has synced to RE but has not yet synced back, thus running the connector one more time will often fix the problem.