Not a complete answer per se but a follow-up to Valorum's:
The First Order & Snoke had no real way to know where Luke had even started, and the Galaxy is a very big place. Consider this:
In ANH, Vader senses Obi-Wan while on the Death Star (within a ~80-km radius) and Luke when he's much closer during the Trench Run.
In ESB, Leia sensed Luke from ...
Luke has "cut himself off from the Force". This apparently makes him invisible to other Force users like Rey, Kylo Ren and Snoke.
“I saw everything,” she said. “The island, and past it I felt the stars singing. I thought my heart would explode. But I didn’t see you. Nothing from you. No light, no dark. You’ve closed yourself off from the Force.”