Plot Browser - Click/Unclick

Hi, a colleague of mine plotted a number of graphs. When he tried to unclick some of them in the Plot Browser he realised he couldn't. The graphs seems to be locked.

We ran the same script on a number of computers, and on all of them, except mine, we couldn't unclick the graphs.

I tried to find out whether there is some property or preference for click/unclick in the Plot Browser, but haven't found out anything.

Would anyone know how to enable clicking in the Plot Browser?

Many thanks, Marina.