During cash-out transactions, users can sometimes undersend the required crypto value by a negligible amount. This can be due to quirks of particular wallets, the user manually entering the amount in their wallet, or the user sending directly from an exchange.
Since the machine is expecting an exact amount of crypto, if it receives less than this from the user, by default the deposit will not be detected and cash won't be dispensed.
However, enabling the Fudge Factor feature will automatically accept deposits that are up to 100 satoshis (0.000001 BTC, 0.000001 ZEC, etc.) fewer than the amount requested.
To do so, go to Global Settings > Fudge Factor in your admin, and set 'Enabled' to 'On' and click 'Submit'.