The amounts or dates don't match.... They do match less the fees obviously... and the invoice shows in the list on other transactions...
The transaction is older than six months... all the transactions I am currently doing are older than 6 months... but all the other transactions from that same time period showed and could be matched
The bank account on the payment isn't the same as the bank where the match is located.... They are the same account
The payment has already been reconciled.... It has not been
The transaction matched another transaction.... It has not been
Multiple currencies are involved, and the exchange rate has shifted significantly, making the amounts very different.... All the one Currency
