Just switched from Jenkins 1.611 to 1.614.
Jenkins 1.615 has the same issue

Jenkins Plugin Manager, in "Updates" tab I have selected
all 3 plugins to update and started the update by clicking "Download now and install after restart" (see picture ), but update did not performed!

I came back to the Plugin Manager. Selected the same plugins and started the update (see picture ). The same plugins appeared with status pending twice and still now any activity!