There are several possible causes to orders not syncing correctly with Shift4Shop's Quickbooks Connector. Typically large batches of orders to synchronize can cause issues. Therefore it is recommended to check for the following:
Order Date Range:
If the range itself is too large, it can cause the sync to fail. A good recommendation is to set the start of the range as the date of the first order you wish to sync over. Then use the last order present as the end of the range. This helps create a more defined range and may help with syncing issues.
Order Number Range:
Similar to Date Range, having a range that is too large can break the sync. It is recommended to start with more defined ranges like the first and last of a particular month to synchronize a smaller batch.
In general, the Web Connector tends to perform better with smaller and more defined ranges than large ones that span for thousands of orders or multiple years.