Recently GPUgrid announced there would be less work available. But don't think that's the only reason you aren't getting work. I had two different machines with 750tis set to run GPUgrid, after a few days of not getting anywork on one, and the other was getting work just fine, I decided to investigate. I guess GPUgrid has upped the required compute capability, again. So the old trust 360.xx won't work for gpugrid anymore. I updated to the newest driver and immediately got GPUgrid work.

So if you aren't getting GPUgrid work, try updating your driver. Still not getting work, your card is likely too old. 5xx series is deprecated for sure, don't know about 6xx.