Hi Diggy,
There will often still be smoke in the box, even when it's not visible out the top "blow hole." Question: Has the food you've smoked been smoky enough? If so, don't worry about it! You actually don't want smoke rolling the entire time. Your meat will "absorb" smoke (actually penetrate the meat) until the internal temp hits about 140. After that, if you keep hammering it with smoke, you risk over-smoking, because the smoke builds-up on the surface (just look to the walls of your smoker for an example of what happens when smoke can't penetrate).
End results is what it's all about! If you see thick white/grayish smoke, you probably have wood combustion, which greatly shortens the length of smoke. If you're using the fruitawood chunks, wrap the bottom half in foil. If you happen to try the smokinlicious wood (which I hope you do someday), you won't need to wrap it. Like I said - end results. If the meat is smoky enough, keep on keepin' on. If not, you may want to increase the amount a bit. We have folks here that say 3 oz is too smoky for a pork butt, but I use 6-7 oz! All about taste!