I too also have the same issue and its alittle frustrating but I now only buy weedkiller that is granulated or concerntrated liquid form and make it up myself (slightly stronger than recomended) I spray the area periodically (weeds visable or not) living in the countryside is a curse and a blessing
Is a just painted wall of do you have lining paper up?
I'd call zinsser uk and see if they have a paint product that can overcome the issue failing that lining paper may work.
Oh dear I have about 24hrs before "she who must be obeyed" will be home and I need to hang some blown vinyl wallpaper (straight match in a brick design) in the hall and landing areas but im concerned about the corners...any help would be gratefully appreciated.