It's just mishapped synchronizations in the Rf....
Do this. There's a little button on the bottom of your mouse and keyboard. You might have to use a pen to hit the button on the mouse. Press down the button on the mouse, keyboard, AND the button on top of the usb reciever. Hold them all down for 5 seconds. Literally 5 seconds. Let them all go at once. The problem is always solved this way.
I was working at Circuit City one day and this guy was returning trying to return his DUO set to us after 5 months of use and complained of the basically the same thing. Said he even paid $50 and showed us the ticket, for a tech over at Allgen (local computer repair shop), to look it over and try to fix it. They told him it was defective and beyond repair for the price.
I showed him how to press down all 3 buttons (tricky with only two hands) and let them go after 5 seconds. They started working like a charm afterward and he stood there for 10 minutes looking at the keyboard and then to me and to the keyboard and then to me with a slack-jawed, dumb-founded expression on his face. Man I get a kick out of it everytime I do that to people who complain about wireless keyboards and mice. You wouldn't believe how often this happens and this sovles the problem every single time.