I'm having trouble running Apple Hardware Test

I have a MacBook Pro Late 2011 OSX Yosemite 10.10.3 - there is an update for the operating system but every time I try to update from App store the App freezes. My laptop is running slow so I'm trying to do some routine maintenance and system checks on it, but for some reason I can not get Apple Hardware Test to work. I have closed all applications on my laptop and shut it down. I push the power button and immediately hold down the D key. The gray screen appears, I'm still holding the D key down. About 2 minutes later - still holding down the D key - my login screen appears. The Apple Hardware Test icon never comes up, nothing happens, my laptop starts as always. What am I doing wrong?