You probably had your hive entrances wide open for the knotweed flow. Well, it's pretty much over. At my location the robbing is ferocious. I reduced the entrances to three inches.
If I see yellow jackets still trying to get in I will reduce it to one inch. I've lost colonies to yellow jackets in the past. It doesn't take long for that to happen.
Of course, it's not just yellow jackets robbing. The honey bees do it, too. They often climb up the outside of the front of the hive to get a higher place to take off from since their stomachs are full of honey.
If you have a weak hive there is almost no hope it will build up before winter. Kill the queen and combine it using the newspaper method with another hive. They say not to combine two weak hives but rather a weak with a strong but I'm not sure about that thought.
Good luck. Just reduce your entrances today and start feeding sugar water 2:1. The booklet Brad put on the website yesterday about nutrition is spectacular. I have to print it out and study it. I think they said to go for four gallons of 2:1 sugar water as a feed in the fall but don't quote me on that.