Coupon Randomly Not Working

Hey Tim - I have a coupon - it is good for ONE time use per person.  I had a major event - and multiple times it would NOT apply the 40 credit.  So I literally have 20 or 30 orders I have to go back and refund that money - which I do not get my fees back -  So I need to know what I may have done to make this NOT work correctly.  The customer was required to use a code for the coupon at checkout - but I personally witnessed it randomly NOT applying the credit?  Any ideas?

Maybe the product being purchased is marked as do not allow discounts. Edit the product and check that option.

Since the coupon  is for once per person, are the ordering on the same device or network? If so, it will also check the IP address and if it is the same it won't allow the discount. 

The allow discounts was correct - this was FOR SURE the IP issue. So If I have an event, where a lot of people could be on the same network - or IP  - then in theory I can't use coupon codes as "one per customer" - I would have to allow multiple - which then also opens me up to the same customer using them twice? yes?  

Yes, it sounds like it.