Debit/Credit card users always get a receipt and there should be an option on the screen where they offer you the option to print one off if paying by cash.

Incidentally, HOP is operated by London Overground and if you can't contact the Oyster helpline, I'd contact them as they're responsible for the machines.

A tip, don't use the options where it says £5, £10 etc. Use the "Any other amount" button and type in EXACTLY how much you want on the card, such as £5.40 etc.

If you can, use the ticket office to top up, however small the amount is.