Hello Saskia,

Try turning on the Multi-colour feedback (under the display tab in the Weight painting tool settings) and going through each joint in turn, more often than not it shows influences that aren't made visible in the black and white mode.

From what I can see of your character though (which isn't much), I would say that you would almost certainly be better off rigid binding the foot geometry to the rig.

Any chance you could post a picture of the full character, it would make diagnoses much easier.

Cheers,

Mat.