One of the biggest issues with fan driven smoke units is fluid will get on the fan itself and actually get down on the motor shaft. The motor will be spinning but the fan will not. The symptoms are exactly the way you describe it.
Solution is to disassemble the engine, get at the smoke unit. Disassemble the smoke unit. remove the fan blade, clean out all the smoke fluid and re-install. You might have to use a very small amount of super glue to keep the motor shaft from slipping on the fan blade.
I have this very issue with a MTH Challenger. Sometime it smokes very well, sometimes it just kind of sits in the stacks like fog.
Other cause might be a loose wire, or bad motor.
