I was able to get free packs of this series, but now I can't. What happened?

To ensure creators are compensated for their work, most series reserve a certain percentage of cards for paying collectors.  That means if free packs are sold out on a series you are collecting, you may still be able to pay for packs if packs are still for sale. If an entire series is sold out, you will have to trade with collectors to obtain cards from the series.