To be successful with herbicides, one must follow directions. Round-up resistance isn't yet in home gardens.

If your weeds are annuals, they should die when treated. You'll have the best results if the plants are in good condition and growing well when you apply the herbicide.

If the weeds are perennials, one must continue to treat -- cut off; pull off; dig out; or herbicide at the right time) every time they reappear. Again, you'll have best results when the plants are in good condition and growing well when you apply the herbicide.

The goal is to starve the roots. Dealing w/ perennials weeds will be a several-years-long project.

Systemic weed killers can burn plants when mixed stronger than the label advises, thus won't go to the roots.

I wonder if she shade is part of the culprit between the front and back of the house?

It would help to know what weeds you've got. There are some that grow from underground tubers, and you can spray RoundUp until you're blue in the face and they'll keep coming up. There are other plants that are not as susceptible to RU as usual - not resistant per se, just different chemistry.

So, what have you got? Pictures would be fine.

Researchers at Iowa State University found, in the 1990's, plants exhibiting resistance to the affects of the glyphosate products, most of which could be home garden "weeds". Other researchers have found genetic problems with Frogs, Toads, Efts, etc. where glyphosate pruducts are used. If these lower forms of life are being changed genetically what does that mean for the rest of us?