What are your weight, and strength settings? If you don't have accumulate checked you will have to let go of the brush and stroke multiple times. Are you using a digital drawing tablet? Do you have pressure turned on and which items do you have it on for?
Hi Julian,
I'm pretty sure the Subtract operation is working correctly and what you are experiencing is Auto-Normalize doing its thing.
The normalization will make all the groups add up to 1.0. And if a vertex is only a member on 1 group, your brush will subtract it to a weight of zero, but then the auto-normalise will normalise it (back to 1.0).
You will need to either disable auto-nomalize or have those verts be a member of more than 1 group.
Hope that helps.