When I click on "Show password:" and enter password then click "Allow" button,
then my cursor just keep loading and password does not show.
(source: shoespost.com)
Keychain looks like it's frozen, only "Deny" button works which will close the window. Same problem with old and new saved passwords.
This happened two months ago after I have upgraded to OS X El Capitan.
New Keychain and reset did not help.
Best Answer
I had the same problem when using my wacom tablet with the Keychain. When using an actual mouse the problem does not occure. I found the following post that explained this problem in detail:
So a solution to this problem is to use a physical mouse to interact with the Keychain.