Promo codes not working after app store approval - "cannot connect to iTunes" then "code already redeemed"

I had my app approved a month ago and generated one code directly after which my colleague was able to use and download the app.

I've been away for a month and now back I need to use some of the promo codes to give out some downloads, however they are not working and instead users are seeing the error message:

"cannot connect to iTunes" and then when they try a second time "code already redeemed"

I've tried calling Apple Developer support who i find rather useless as when they take calls they always have absolutely no idea what the problem is and then ask you to provide a vast amount of info so they can investigate, which often results in no reply.

I've read through the forums for others experiencing this in the past, but the solution to Release the app and set a date for release is not really an option as we don't know what the release date is currently.

My first question is - does anyone know an alternative solution. What's the point in promo codes if they cannot be used in the intended way?

And secondly - if I released the app and set the release date, can I change this to a different date later on?

Many thanks for your support.