sounds to me like they are not letting it fully heat initially, then possibly pulling too much air in, cooling it just enough to keep vapor production low, but allowing enough heat to make the unit hot. I did this a few times over and could not figure out what I was doing wrong. On a cold start, try holding it for 20 sec. then start a slow steady draw, then as you taste the vapor and feel the heat building up, increase the draw to pull more hot air through. I know it can be frustrating when it seems like it's getting hot but not producing. Now that I have my timing down, I get good solid hits every time, until the load is spent or the batteries get low.
Always try to start a fresh load with fresh batteries, until you get the hang of it.