The SPI (System Performance Indicator) on my Lux Commercial by Aerus upright didn't work at first, as I discovered when I pulled out the jam-packed bag. The bag full bulb didn't illuminate at all—not even a flicker! I also noticed a fine coating of dust both in the bag chamber and on the outside of the filter bag.
So, going by the satisfied reviews I'd read on Vacuumland, I decided to purchase some DVC brand filter bags from a thrift store to see if they'd work better for me, too, rather than the blue Home Solutions brand that I'd been using.
That made all the difference in performance. Along with improved filtration and air flow the SPI bag full light now illuminates when the bag is about ¾ full.
Btw, I contacted Aerus to see if I could exchange what I perceived to be defective filter bags and was told they have no agreement with the supplier to honor the request.