I am having an issue where registered customers are able to use a coupon code but customers that are not registered cannot. Basically the customer inputs the coupon code and it looks fine but when they go checkout and type in their info, the coupon disappears. Can anyone help?
In your shopping cart price rule or catalog price rule, my guess is that you only have the signed in customer group selected for the coupon.
For users that aren't signed in, the coupon may be invalid.
If that's not the case / doesn't fix it, then there's likely a coding issue with a third party extension as a conflict, or a design file is stripping out something that should be there.
If that doesn't fix it, I'd run a code audit on the site to see what might have been altered: http://www.customerparadigm.com/magento-development/free-magento-code-audit-report/ (Note: this is from my company's Website.)