I'm not sure this totally applies here but we have found that when the match button doesn't appear, the invoice payment was sometimes inadvertently received in the wrong currency bank account. Check the Payment record and make sure the right bank account is selected for the payment. Switch it to the correct account and the Match button will appear.

We sell in USD and CDN and utilize USD and CDN bank accounts. We have seen this appear a couple of times and it was just the wrong bank account matched to the invoice payment currency.

Kyle