Obviously they get many such frustrated emails and now just send a standard response, this was the reply I got last week which bears a marked resemblance to yours:-

Dear Mr -----

Thank you for your email regarding your award ticket request.

We fully understand your frustration in regards to the unavailability of redemption seats on your desired flights and we do apologize for any inconvenience this might caused you.

As you may be aware, redemption seats are very limited and subject to availability. This availability could vary depending on seasonality.

As much as possible, we would like to provide you the flights that you desired, unfortunately, we are still unable to find redemption seats due to peak season.

In some cases, seats would open and become available after some time. To avoid missing out on any available opportunity we would suggest that you keep checking with us from time to time should the seats become available.

We appreciate your understanding and consideration in this matter.