I don't think you are doing anything wrong.. Sometimes Photo Stream needs a jump start.
On the iMac open System Preferences > iCloud
Deselect the box next to Photo Stream then reselect the box then restart your Mac.
On the iPad go to Settings > iCloud
Toggle Photo Stream off then back on then reset the iPad.
Hold the On/Off Sleep/Wake button and the Home button down at the same time for at least ten seconds, until the Apple logo appears.
Thankyou. The kickstart worked and the 656 iPhotos which were stuck, supposedly loading, have gone and I was able to drag the previously imported photos on to Photostream which started to upload them (rotating wheel next to Photostream) and then appeared on the iPad. So far so good, thought the upload,as normal, is very slow eg 2 minutes for 3 photos. Then I tried to drag a smaller no of photos from the 'Last 12 Months' to Photostream with no success. The green + did not appear over Photostream and the Photostream Page has 'Unable to connect' at top RH corner. Went through the deselect/restart routine again , the unable message is gone and it now uploads OK! Is this yet another unadvertised and unpublished "feature" of iCloud? Or one of many bugs? Thanks anyway even if it a tiresome but doable fix.
Happy New Year!
I had forgotten the usual restart/reset solution, having been using, until recently, old stable stuff on 10.4 on a G4. Perhaps I should have remembered that it is exactly 50 years ago, 1962, that I started on computers. Working as an operator on LEO lll, a UK mainframe machine that filled a large hall.It had a ferrite core memory of 16k, but was multitasking. Restart was the usual solution to many problems. Plus ca change!