Shutter won't fire when using rear screen

One of my little seasonal rituals is to take some photos of the first daffodils that pop out in my yard to celebrate the coming of spring. Now that I'm in my 60s, getting down on the ground to compose those shots, or for any other reason, and back up again, is becoming increasingly difficult, so now that I have a camera with a flip-out rear screen, my EOS T3i, I decided to give that a try in lieu of the optical viewfinder. I've never been one to shoot with the camera at arm's length, so in the ten months I've owned this camera, I'd never before thought to compose my shots on the rear screen. After activating the screen with the button to the right of the viewfinder, I was able to compose, and when I pressed the shutter button, the lens focused and stopped down, but the shutter wouldn't fire; however, when I turned the screen off and resumed using the optical viewfinder, the shutter worked fine. Did I miss a step, or is there something wrong with the camera? I'll admit I'm nowhere near mastering all of the camera's functions, and on my EOS XSi, I couldn't even find a way to activate the rear screen to compose a shot. Also, I only tried this with the 18-55mm kit lens, so I may try a different one today, just to eliminate that possibility.

 

So, I tried it a little while ago with different lens and everything worked fine. I then put the 18-55mm kit zoom back on, and it also worked fine. Not sure what the problem was.