It's the temperature differential that makes the engine run. When the lower surface is heated for an extended period of time, the upper surface may slowly heat up too, due to the internal hot air exchange. In this case, it is normal that engine speed decreases after a while unless the other side is cooled.
The ball bearings are sensitive to contamination from foreign particles. Keep dust away from the engine. In case the ball bearings are contaminated, they can be rinsed with alchohol.
Large heat sources (bigger than the engine itself) will eventually heat up the entire engine, because it is "submerged" in warm air, eliminating any temperature differential. There needs to be a temperature difference between the top and bottom plates for the engine to work.
