We only allow 3 boards to be printed in front of the pick and place equipment. It is important to keep the abandon time to a minimum.
You did not mention the paste that you use. Do you use no-clean or water soluble? Do you use lead free?
Solder paste is hygroscopic which means it will absorb or expel moisture. Moisture can cause problems such as slumping, solder balls, splatter, and component popcorning. If your environment is too dry you paste will expel moisture (dry out). This will cause other soldering issues.