I don't think the room gets immune, possibly you have air flow issues. For my testing, I have come to the conclusion that I cannot get adequate HT results testing in some areas of my house. For instance in my living room I have air ducts on each end of the room and if I position my candles in that air flow area it seems to suck the scent right out of the room. I have found a few "spots" in the room that do work though. I have also found that testing when the AC is on doesn't work well for me either. Now in smaller rooms like our bedrooms and my office, I don't have a problem. With that being said, I went thru testing woes when we tore down our old house 5 years ago and re-built our new one. The ceilings are higher and the rooms are larger which put me in a situation where I had to re-structure my testing parameters. Anything and everything would throw in my old house, so I was very challenged when I began testing in the new house As far as the size of the candles . . .I cannot get one jelly jar sized candle to fill a large room to my satisfaction. I need wider mouth jar opening that provides a larger melt pool. That is my preference - I like my HT to hit me in the face when I walk into the room and float throughout . . HTH