I've had this problem too.
Last time it happened, I had to submit a ticket via the webpage. It was annoying that I had to do so, but once the Account Support bod got in touch it was dealt with very fast (Thanks again Dyonysus!) Even though my enquiry was made on the last day of the bonus points week, but wasn't picked up for a couple of days, there was no problem with awarding me the bonus points amount I had been attempting to buy.
Unfortunately though, I am STILL having this problem - happened again when trying to get some points yesterday