Jeff, for starters using the Lionel Premium smoke fluid makes a real improvement over the regular stuff.
I too, encountered an instance where this type of smoke unit was underperforming. If you look over the smoke unit, there is an air chamber with a sort of piston that goes in and out. This can all be easily snapped apart. Inside the air chamber, there's a spring. I stretched out the spring a little bit and put it all back together and the issue was solved.
You also want to check and make sure the side rod that engages the lever on the smoke unit (as the engine comes) is pushing that lever of the smoke unit properly. A slightly bend side rod can interfere with this. This is where stretching the spring assists in pushing that side rod back and getting a nice puff of smoke.
Lionel has also added a resistor to the more modern steamers with LionChief circuitry. When I've used these smoke units in an earlier made loco without the LionChief circuitry, I remove the resistor.