Coupon Code works for some users and not others

I have coupon codes that have been working just fine for months until today. (No changes made to the site recently that I'm aware of) Now some users cannot use a coupon code while others can (same coupon code). I checked the limiting parameters for the coupon codes that are presenting problems, but to no avail. There were no parameters that should be preventing the use of these codes. I logged in as a site user and the coupon codes worked. Then I logged in as one of my site users that are having problems and the coupon code does not work when logged in as that user. Again, I have checked all the limiting parameters for the coupon's use and they should not be preventing these users from using these codes. I've tried creating new users and checking out with the code and the code works. But for a handful of users (some created today and others in the past) the code doesn't work. I cannot seem to find a common thread between any of the users it doesn't work for and the users it does work for. I need some expert assistance to this very frustrating situation.

Hello,

Try going to awocoupon->configuration->error messages tab and change all the errors to something identifable. Then try the code again in an account that does not work. The error message should then give you an idea of why it failed.