It's likely that a tax code or a service item that was active when you first setup your LMN/QB Sync has been deleted or deactivated in your Quickbooks account.
Re-run the matching wizard for Service Items and Taxes and try the sync again (step #4).
If that doesn't work, run an estimate review report on the estimate that is not importing and check to see if there is an old tax code in there somewhere.
You can change your tax code on the estimate by clicking on the purple pencil on each line item. Change the old tax code to a new one and click save.
Try to run the estimate import again in the sync wizard.