How can I reorientate the screen?

When I press the part of the screen to play a game, nothing happens. Sometimes the 'pictochat' and 'DS download play' flash, even though I am pressing the game part of the screen. Is this a reorientation problem? I have just fitted a new screen protector; could this have anything to do with it?
DS Lite; model number:USG-001

CHRISTINE SELWAY
January 2011