Q: iMovie says waiting and will not load

I have just bought imovie from the App Store - but it doesn't seem to be loading, it just says 'waiting', it won't allow me to press the 'install' button, as it is greyed out.

I have rebooted a couple of times, but still the same problem.

I'm running OS X Yosemite V 10.10.1 on my iMac 4GB Memory.

I'm at a loss, so have turned to the very helpful Apple Nation for some help.

Pete