Doff Weed Feed And Mosskiller

[malagabob]

Bought and used some of the above. I have noticed plantains turning black but all dandelions seem un effected. Do I need to give it more time.

[alavahalf]

Doff contains glyphosate and should see off most weeds.
Glyphosate doesn't kill weeds overnight, it is a systemic killer and goes down to the roots over a couple of weeks and you should see the weed discolour before giving up completely. I find it works faster in dry weather, when there is less moisture in the air.
Give it a bit more time and then re-apply.