Once a Fx is in the timeline, you can no longer position it based on the timeline scrubber as the scrubber refocuses to the Fx when selected. You can do a couple of things:
1) Place timeline scrubber at desired Fx start location, use Shift M to define a timeline marker. You can then move the Fx to start at this marker and it will snap to marker
2) With Fx selected, you can use Alt L/R arrows to nudge Fx one video frame at a time left or right
3) You can zoom in timeline and move Fx one video frame at a time or change the duration one frame at a time.
4) You can use right mouse click on Fx and use set duration
Jeff