Simple MOSFET switch circuit PCB design in altium.
So let's get this out of the way first, I usually don't like routing a PCB while the rat's nest is enabled. I think it's more confusing than anything. However, if you want to display it you can go to the "view" tab and find "connections" at the very bottom of the drop down menu. You can then choose to display all, hide all, or hide certain nets. Of course, for the rat's nest to appear, the pins have to be connected in the schematic. In your PCB, you can double click on the two respective pins and check what net they are assigned to. 

Finally, if none of this is helpful, maybe check the pin assignment in the schematic file? It has happened to me that I connected pins to the wrong nets, simply because I thought it was obvious where the connections had to be made, based on pin locations in the schematic.