free spin-credits do not appear in slot-machine-screen.

are there any conditions that must be met?

I don't think there are free spins.

If you place a "max bet" then one of the thinks you can "win" is free; as in free cards.

If you play the game, the top right of the screen shows the percent discount available for the bet you are making. For a one credit bet, you can purchase the cards displayed at 15% to 55% (which ever percent shows up) for example.
If you make the maximum bet of 10 credits, the discount includes, at the top end of the discount, Free (35% to free) This means you have won the cards on display for free. Not just any card, but the cards on the display of the slot machine.

good luck.

From the last TGIF-post: "Good Luck, today is Friday the 13th! 13 free credits of spin are waiting for you on the freshly unlocked Winter Slot Machine!"

There is even a screenshot, where free "spin-credits" are visible.