To be honest, its a pretty popular thing we encounter with machines we recondition. Usually non-clutched ones.
We haven't managed to nail it down to any one thing, but as daft as it sounds, we just have to tweak it to reduce the suction. Removing the sole plate seal is the usual way. It just allows that bit more air to pass by and reduces the suction just a little. Other than that, we fit a more worn brush roll; again just to try and reduce the efficiency of the machine.