Just to be sure, I thought I had some UNO R2 units but it turns out they were the original 'R1' versions. Which require an additional step to before it can be put into DFU mode. Just shorting the RESET to ground alone will not work with this version of the boards. See image attached does your board have the two pads circled in red? If so, those need to be shorted together at the same you short the RESET to GND.
