I'm not trying to be a wise guy, but did you press the temp button first before pressing the up/down or the time button before pressing the up/down?
I was thinking the same thing as Kevin here. When you press on the oven set time and then the up or down arrow, do you hear a little snap and feel a tactil depression when you press on the button area? Some have had problems with their fingers either being large and not pressing firmly in the area where the button is to depress it fully, or are just not pressing hard enough to activate the buttons. Just a couple of thoughts for you to try.