Siri bypasses locked screen

if you followed this procedure - Settings>Passcode>enter your Passcode>Allow access when locked>Siri>Off - and that did not work, try turning Siri off, reset your iPad, turn Siri on again and try changing the setting again.

Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.

if you don't use the latests ios version upgrade

Just so you know, I also have an iPad 3 running iOS 7.1.2 and I tested it to be sure, and it works fine on my iPad. I wanted to try to rule out anything weird with the iOS update.

You could try resetting a ll settings. That does exactly what it sounds like it should do. It restores the iPad settings to out of the box condition. You don't lose any data, but you do have to re-enter all of your privacy settings, app preferences, wallpaper, etc. Settings> General>Reset>Reset all settings.

I would hope that resetting all of the settings would fix anything that might be corrupt that could be the cause of your problem. I do stress the word "hope" because I don't know for sure that it will work.

I understand. Just so you know, I have reset my settings more than a few times and it has worked for me every time I tried it. It's not the end of the world. It is a little bit time consuming, but it pales in comparison to restoring a device - and I have done that 3 or 4 times with success.

Good luck. I hope that you find a fix soon.