I am not sure how this affects the Slingbox. Unless you're saying that the record button of the Slingbox's virtual remote is not responding as you expect. Then I would recommend in testing the DVR's physcial remote control to make sure the problem isn't with the DVR itself. If that works, then try reconfiguring the the respective video source. This process will reload the remote files onto the Slingbox. Maybe the current file forgot what it does.
And if that fails, let us know the exact manufacturer and model of the DVR, maybe there's a custom remote file that you can use.