Some of you cannot smell them because you get used to the scent. I can't smell many of mine but people come into my home and comment about how nice it smells.
You can tell when your warmer isn't working if you feel it and it isn't warm.
I was told by an SA in B&BW that the warmers are only designed to last about 6 months and then have to be replaced. He said they get a lot of complaints about that.
I detest any of the Glade plug-ins. I've tried tons of the scents and they all smell cheap and chemical to me. The Air Wick plug-ins are nice and I use those often. My favorite are the Febreze ones. They bring out a couple of scents at the holiday that are really nice. I stock up on those.
I have found with the B&BW bulbs that some of them smell heavenly and are strong, some smell nice but aren't very fragrant and some smell disgusting. What I don't like is that they put the corresponding candles out to smell before buying the bulbs. However, the scents aren't the same. I love the Winter Candy Apple candle but the bulb in that scent was nauseating. Same with warm vanilla sugar. Love the candle, hate the bulb.