Altium Designer 19 net name hidden on tracks
In the PCB view, in 2D design view:

Press 'L' to bring up the View configuration dialog:



Select the 'view options' tab

Scroll to the bottom and you will see the view options. The ones in blue are visible. the ones greyed out are not.

Changes take effect immediately, so you can watch the view toggle on screen as you turn these options on and off.