Same thing has happened to me, so called the number above.
The guy on the phone was very helpful and confimed that, as a season ticket holder, I should be able to purchase individual extra tickets for the home games against Newcastle and the Blunts (rather than the ticket bundles open to all other registered fans).
Despite this, when I log onto the online shop it only allows me to buy the ticket bundles for these games, so he advised I try again tomorrow and, if it still doesn't work, I should phone back and they will do it over the phone for me.
Still seems to be a few teething problems with the online shop....