While online ad inventory is increasingly added to TV buys, upfront buyers are still not flocking to cut checks, reports Adweek.

Large content producers and distributors, including Microsoft and AOL, hold upfront sessions just like TV networks. Those sellers add online inventory, often from repurposed or extended TV content, to their sales pitches. But buyers have not flocked to buy inventory early, in part because they seek more flexibility in online buys. And scarcity, which drives the costs of TV ad sales up, simply doesn't apply online.

Part of the problem is that video content is so scattered that portals cannot lay claim to audience figures they once could. Those numbers, combined with other factors, put power firmly in the hands of buyers.

Even major portals like Yahoo, which also experimented with holding upfronts, are abandoning the practice for lack of buyer interest. Instead it is holding small one-off meetings to sell what it can, when it can.