Perhaps that's because people who have already bought one of the sets have realised they can make lots of money by selling it unopened.
One family has already spent over £300 on a Hogwarts Castle that they bought on the internet.
In fact, CBBC Newsround Online has discovered that there are more than 60 Lego Hogwarts sets on sale on the internet in Britain and America.
Lego: working 24 hours a day
On an auction site a person puts something up for sale and then after a chosen period of time, the person who offered the most money gets the toy.
Lego say their factories are working 24 hours a day, seven days a week to try and meet demand.
But they are not sure when they'll be able to make their next big delivery of the sets to toyshops.