Coupon is Removed After Quantity Amount is Changed

I have a slight situation where my client is trying to sell a product of 6 at a time. So the goal is to sell 6 of X for $20, 12 for $40, etc. The coupon is working just as planned but then we found that when you add 7 items the coupon is removed.

Each individual product is $5
Buy 6 of the product and get a discount of $10 for a total of $20
Add a 7th item the coupon is removed and each item is charged at $5 each

Is there any way to keep the 6 for $20 coupon for the 6 items and then charge the 7th item by itself?

Each individual product is $5
Buy 6 of the product and get a discount of $10 for a total of $20
Add 7th item and the coupon stays for the first 6 and the 7th item charged individually for a total $25 instead of $35.

Thank you.

the 6+ then becomes 12+, 18+, and so on. I left the 7th item as 0 because we only want 6 items to be discounted. I could be that this is the problem but again, we just want it to apply for every 6 items and if they have more to group the first 6 in the coupon and charge the 7th item at regular price.