This happens when you are betting on the slots. The Bet feature allows you to double or triple your winnings, but it also uses multiple Spins each time. When you have one spin left and use the "BET X2" or "BET x3" feature, you will not be able to spin.
You can cancel the Bet feature by clicking on the "BET X3" button until it cycles back to "BET X1".
The bet button can be found above the spin counter.