Why can't I click on video to move/resize it anymore?

I'm new to Pr, and am currently testing it out on the trial period of CS6.

I've already made one sequence where I loaded in 4 videos, then simply clicked on them in the workspace to move/resize them into each corner of the screen. It was dead simple, like moving/resizing images on Word or PowerPoint.

Now onto my second sequence, I again loaded in 4 videos, but it wouldn't respond in the same way when I clicked on the video in the workspace (white frame with dots in corners/edges with scope in centre), so I can't move or resize it.

I tried with just one video and still no response.

WHY HAS IT STOPPED WORKING, I ONLY NEED THIS RELATIVELY SIMPLE FEATURE TO WORK?

IS THERE ANOTHER WAY TO REPOSITION/RESIZE THE VIDEOS APART FROM USING THE MOUSE CLICK METHOD?

Please help :'(