In an article from Circuits Assembly Magazine, solder paste is determined to be the key factor in proper QFN assembly. Joseph Ameen and Gilson Geralde mention that "If too much solder is applied to the ground plane, the part will float, resulting in poor connections to the I/Os. If too little is applied, insufficient grounding will result."
Although this may not be groundbreaking, it's the reason we shoot for 100% transfer efficiency with our solder pastes. Many solder pastes vary between 60% to 110%, while the best solder pastes will see tighter than 80% to 105% transfer efficiencies.