I know how to do this, as I have for a year or so = I have standalone QBks 2019 Mac - download trans. each month or more often. For example, just downloaded trans. from 3/22 - 5 were supposed to come in, 5 show up in the bank online listing of items which could be downloaded == but only 4 come in and 1 does not == this happens each month without any pattern. I have asked my banker and they have talked to IT and their folks and said to check with Qbks so I am...or trying to.
QuickBooks imports bank transactions in a file format called OFX. Each transaction in an OFX file has a unique transaction id. Below is an example - in this case the transaction ID is the number following <FITID> or [removed]. The numbers _must_ be unique - they can never be used in the same bank account for two different transactions. If they are that's a bank error (Collect $200! No, just kidding!)
QuickBooks keeps track of the transaction ids that have been imported and what you've done with them. For instance if you delete a transaction from the download transaction window QB will track that FITID number. Next time you pull transactions from the bank that same transaction may get pulled down again. If it is, QB will see that you already marked it "deleted" and it won't show up in the list of downloaded transactions.
So, since you are using WebConnect, you have the .qbo file. Open that in text editor and find the transaction that isn't being imported. You can change the FITID for that transaction to something different (make it wildly different - like add 7777 to the end of it) and try importing the qbo file again. If the transaction now comes in the issue is the original transaction id.
The question then is whether you've already added that transaction to the register or if you've marked it for deletion. In either of those cases QB is working as designed. It has to do this to prevent duplicate entries and ignoring your commands to delete a transaction.
The more insidious case is if the bank isn't marking the transactions uniquely. In order to determine that you have to search all your .qbo files and see if that transaction id has been used previously in one of them. Spotlight can search them so just typing the transaction id in Spotlight may tell you - if you've kept all your old .qbo files and haven't deleted them.
You may well be right but what happens is the exporting of the transactions is in a QBO file
when I go to upload to my QBks I do see the language you are showing - I was able to open it in text edit and you can see the way it is -- this is the transaction that didn't make it in - the other 4 did. - I just saw the other reply and will try that to isolate the problem
Getting Closer. I opened the QBO file in Text Edit and changed that number - saved the QBO file and then tried to import it again [I had already deleted the earlier missing transaction] -- the 4 that made it in before were not imported, of course, but the missing one WAS - I then just did the import as usual.
So the issue is the # in the FITID line apparently has appeared before? I did a search for that old # of the transaction - 526334 - no QBO files but some sort of article on vegetables!
If the vegetable is rutabaga that's your problem. QuickBooks hates rutabaga.
Seriously its difficult to determine the cause of a dup FITID unless you happen to have a smoking gun showing it used twice. But you can proceed with the fix you have. Keep your old .qbo files in a folder in case it happens again.