When I try to withdraw money from a cash machine, it tells me I don't have enough available funds.

Some cash machines charge you for using their services, for example if you're using your card abroad. This fee is added to the total amount you're trying to withdraw, which could result in rejection due to your available balance. If this happens, you will see a transaction amount on the screen that is greater than the one you asked for example; 51 USD instead of 50 USD. In this situation, you need to withdraw a smaller amount so that you stay within your available balance once the extra fee has been added on.