AdBrite having trouble filling CPM spots?

One of my ad zones has 5,758 impressions and they’ve only shown 521 CPM ads. This is about 90% US traffic, 5% Canadian, and the rest elsewhere. Am I doing something wrong or are they just running low on CPM ads and filling it in with the same 5-6 text ads that they always put up.

See original post by Phooey