Unless there is a "bug" that is causing it to behave otherwise, when you select the "Not a Match", it is unmatching it from the current existing transaction, but then it is rematching it to the next existing transaction of the same amount.

If you click "Not a Match" over and over and over again until you run out of "matching" transactions of the same amount, then it will finally show as an unmatched transaction.

BE CAREFUL! Once it becomes an unmatched transaction, you need to select the payee and account and click "Quick Add" before you do anything else, otherwise it will get auto-matched again and you will have to start all over.

Thanks again Intuit!