The screen above the oven allows particles up to 0.04 inch (one millimeter) in thickness to enter the oven. All materials larger that this size are directed away from the oven.

An infrared beam across the opening of the oven verifies that the particles are entering the oven. It did not detect any incoming dirt particles.

The oven is connected to the Thermal and Evolved-Gas Analyzer (TEGA), which is designed to analyze the composition of Martian materials that enter the oven.

Unfortunately, all the soil, about a handful in total, landed around the oven, and not inside it.
 
Phoenix mission manager William Boynton, of the University of Arizona (Tucson), states that he and other members of the Lander team are investigating the problem. They are also determining the best way to coax some of the dirt sitting around the oven to move into the oven itself.

What do the mission managers think went wrong? More info follows.