Rosie, are you trying to clean the sensor the manual way, or the automated way?
Automated way: if th ecamera has a sensor clean function then it attempts to clean off(shake/sonic blast) the biggest of dust particles, and may dislodge the smallest ones as well.
Manual way: Using a wet sensor cleaning product is usualy the only way to remove 100% of stubborn dust particles.
I think dust is a random event that adheres closely to the laws of Chaos(a universal law that says something to the effect that almost anything goes.
If you haven't tried a manual sensor clean, then if you come along to the next AP meetup on the 18th, I can bring my cleaning stuff and show you how easy it is to actually clean the sensor properly.
We may have to go somewhere isolated, private and quiet tho!
(lol!... the hard part is trying to find a dust free environment, with no draught).
let me know if you're going to make it and whether you need help with it.
Once you've seen how easy it is to do, get yourself a cleaning kit(approx $50-60 or so) and it should last you about 3-4 years(on average).