Cant upgrade from Lion to Yosemite??

I downloaded it, clicked on instal, entered my password but then nothing happened?? And every time I click on instal it keeps asking for the password but beyond accepting it nothing happens. How to fix this? I did the same upgrade with my 2011 MacBookAir and had no problem. Deleting and redownloading Yosemite didn't solve it either.
(for now it has to be an upgrade, not clean instal)

btw. I mean MountainLion