Jazzie, MY feeling is that you're not applying enough pressure with the sensor swab to actually "dislodge and pick up" that stubborn bit of whatever is on the sensor. Imagine it as a bit of stuck-on egg on a breakfast plate that has dried until dinner time...it's not just going to come right off, not without a bit of elbow grease. (I say this with all the confidence of a dishwasher located 2,000 + miles from the plates...lol, but I have had a few stubborn spots that took, not surprisingly, a lot of pressure to get to come up.) I do think gryphonslair99's Warren Buffet Method of sensor cleaning is also a viable option. It's the method the camera makers always recommend.