Coupon code is not working most of the times

Hello

The coupon becomes not valid some times - I have received 3complaints so far this week. 

I used all possible coupon code formats. 

For example of this form: https://form.jotform.me/seoways/555-network

i used these codes: KILLER20  , Killer20 , killer20

but im receiving complaints 2-3 times every week  that coupon is not valid when the clients fill the form. Can u please look into it and let me know what couldbe the problem

Thanks

Apologies for any inconveniences caused. I have tested the coupon couple of times and could not replicate the issue.

In case the error occurs would you kindly request for a screenshot and share with us?