Last Watchstone Wont Drop

I have 4 watchstones socketed in my last region but when i run maps there no crusader influence spawns. as soon as i ran a map in a different area, crusader spawned immediatly.

Assuming you mean that you have 31 out of 32 watchstones, and "last region" means the one region where you still have a red watchstone available to drop, and the map you are running is the correct tier for four stones in that region...

I imagine you've figured out that there is only a 50% chance to spawn influence on any attempt, starting from 20 stones. So it's quite possible to run a bunch of maps and not get influence.

5 misses in a row would be bad luck. 10 misses in a row would be really bad luck, but still probable enough that it will happen to a few people. Keep running maps in the region. It's the only place you're going to get that last stone.