I have a problem similar to the one you have, i unable to extend display to my second monitor but i could clone monitors without any problem.
The trick i found, for now, to extend my display is to update the driver
for my video card. At soon, i install a new driver, my display come extended and work fine. Unfortunately, at soon of i reboot, i lost the extended display and i need to update the driver again (i just go back and for with 2 drivers
). My video card is a HD2600
Try this trick for see if you have my problem.