Sorry to hear your automatic discount hasn't been working. To clarify what @jam_chan asked, did you set and end date for the discount, perhaps? If this is the case, you can go in and adjust the end date, or remove it all together. This is Step 9 in our help document on setting up Buy X Get Y automatic discounts.
Another reason this may have stopped working is if you set up another (different) automatic discount. It's only possible to have one automatic discount active at any time. If you activated a different automatic discount, this will make your previous automatic discount inactive.
If neither of these options do the trick, let us know. A reminder that the customer will have to add all products to their cart for the discount to work - including the free product. Make sure to run through the steps in the help document I linked and see if there is anything else it may be missing.