Our mash tuns endure a complex manufacturing process to bring the inner and outer walls together to form the vessel. The lid is also double wall insulated. As a result of the manufacturing techniques, there are some orientations where the lid may not sit completely flat. In fact, the lid is "fit" to each vessel and will lay perfectly flat in the correct orientation.
Each vessel and lid have a sweet spot, where the lid mates perfectly for form an air-tight sea. If you find that your lid does not sit flat, start by rotating it 20-45 degrees and to see if you can locate that exact spot. Then, note where the handle is positioned relative to the front logo so that you can easy find it again in the future.