Hi Lynn - there are a couple of scenarios in which this could occur. If the slot was booked and either cancelled by the user or you or cancelled by the system because it wasn't confirmed and the user re-books the slot, it will still count against her usage. We do this so users can't reserve a slot, cancel it, and then re-book it for free.
I do see a potential issue though. The free booking window starts at whatever the current time is and ends, in your case, 36 hours later. So, if you have two slots of 12 hours each and the end of the slot you're trying to book is after the 36 hour period, you wouldn't be able to book it without it costing you a slot.
Seems like a better method would be to allow the free booking if the start time of the slot is within the free booking period without regard to the end time of the slot. Seems more intuitive. I'll put that on our dev list for this week.
Let me know if you think it's some other issue and I'll be back in touch when I have the new code uploaded.
Thanks,
John