Chuck, I have seen similar results during seasons with alot of rain where the grass will grow back faster. While it is frustrating, this is still the best product I have used to kill grass/weeds.

From my research and own experience with this product, Glyphosate enters through the leaves. It kills the plant by blocking the shikimic acid pathway preventing it from making a vital protein. This means you have to actually spray the product directly onto the green/leafy vegetation for it to take effect. As you can see in my photos there is a very clear line about 1-2ft wide directly under the fence where the product was sprayed and everything outside of that area actually grew quite well.

If you use this product be careful not to spray on a windy day to avoid it from blowing on nearby plants and have patience. It takes a few days to see ANY change to the plant before it completely dries out and dies. Once it takes hold, it is very effective.