I have the same problem. If you're interested, I could list the various things people have told me would fix it.
Belt tension is the most common one, though I've yet to have anyone come up with a way to describe how tight it should be other than "not too loose and not too tight" but never any way to tell which it might be. I was told the more important part is that it's equal, which I have measured with Ian's tension gauge.
Extruder flow rate calibration has been mentioned. This is the only one I haven't yet ruled out. I tried it once but after completing the steps in Ian's two blog posts (this and this) on the subject, I ended up with prints totally failing; there's some variable that those two don't discuss which must also be off, that mostly cancel out with the flow rate problems, but I haven't taken the time to find out what yet. By the way, this is supposed to be something you do separately every time you load new filament(!).
Bed levelling has been mentioned, but I don't see any off level on my bed.
Increasing flowrate on the Manual tab to 101% was supposed to possibly fix it, but it didn't.
In my experience, ask about a problem like this ten times and you'll get at least seven answers. These printers have so many variables (software settings, hardware calibrations, filament quality, ambient conditions) and so many of them interact with one another so whenever you change one, five others have to change, which is why it's so hard to adjust anything. This is probably the #1 thing that will have to change before these are consumer-friendly, isolating the variables from one another.