Hi Frog. One possibility is that you have turned off displaying the ratsnets. Check your layer colour/drawing configuration dialog.
The connections may have been hidden at the net level. Use View | PCB > Nets, right click on all nets and show connections.
Much less likely but if using single layer mode there is the option to hide ratsnets not associated with that layer.
It was of course the very first of your suggestions. I had never noticed that option until now.
Thank you Peter.
I've encountered a strange problem where the ratsnest (i.e. lines showing unrouted tracks) has disappeared. A DRC lists the unrouted nets and shows them up when I zoom in. I'm not aware of doing anything to cause this.
I've tried clearing and re-importing the netlist, but that didn't help.