I am confused by what you are saying is free play.
Starting with no coins / no credits - Can you hit spin or pull the handle ? and if so does it spin the wheels and pay wins ?
Or does it just allow the handle to be pulled but no reel action ?
Every now and then you put a quarter in the machine pull the handle and play the slot machine, then you can play again without a quarter over and over again. If you hit the spin button it will lock the handle, and play normal again. The spin button works every time, the handle is the issue every now and then.