I do not see raid tracer lines while doing raids. It doesn't show at all , even during raiding persian positions.

Any idea why it happens ? 

No. I get the same problem but the game is so full of glitches I put it down to being yet another one.

It's smallbug, one of mine teammates have that problem too, but he don't mind much cause he just need time tracker, he can play without line tracker. As I said, it's small bug and it's on your account and your city i guess. Please report this problem to our Support Team , they should take care of that after looking into your city.