I recently faced a problem of a total lack of briarthorn in the Barrens, which are known as a location with the highest number of spots of this weed. I ran the whole location twice and haven't found even one spot of a god damn briarthorn. What the hell?
Go to Silverpine Forest and Hillsbrad Foothills, there's plenty of Briarthorn there. Nodes are changed sometimes to prevent bots from farming them I believe. Could be that they were wrongly placed aswell.