What is the status of the Coupon problem?

My marketing campaign for postgraduate students returned a great many wanted results to my site - only to find out that the coupons feature is not working. It says: "Sorry, but we do not seem to have that code on file, please try another."

My version of Membership Pro is 3.5.1.8

I did run an initial test with existing students whereby based on the coupon they are granted a particular subscription access. That worked and so I proceeded with my campaign.

None of the subsequent coupons that I registered, I found out only yesterday, worked!? And I don't now have time to go into the webdev code and detail.

Ran the database tool already - everything is okay.

Reading up on all the related questions it seems that you are really having problems with the Coupons feature?

The coupons feature in 'Membership' plays an integral part and reason for choosing WPMUDEV. And to think I'm only using it currently at a basic level (not on a payment level as with many other queries).

Is there any definitive solution?