I have the following machine and have kept it up to date with OS updates.
Mac mini (Late 2014)
1.4 GHz Intel Core i5
16 GB 1600 MHz DDR3
When I clicked on "update" to OS X El Capitan from the App Store the downloader started and finished but it has since stalled. I can't even download the OS again.
This OS shows in the "purchased" pane but the button on the right side of the window shows as a greyed out "downloaded" button.
Any tips?