Thanks, that make sence. I forgot to add in the second vector the my.position so I think that the trace was form the center (green lines) instead of from the entity.

Now I change the script as follows (again only the horizontal moving is showing)
Code:

---

	// trace to the right
	vec_set(trace_right,vector(my.x+scan_dist,my.y,my.z));
	dist_right=c_trace(my.x,trace_right.x,ignore_me|use_polygon);
	if(dist_right==0 && move_left==0)
	{
		c_move(my,vector(8,0,0),nullvector,ignore_me|ignore_maps);
		// second trace after moving
		dist_right=c_trace(my.x,trace_right.x,ignore_me|use_polygon);
		if(dist_right==0)
		{
			move_right=1;
			move_left=0;
		}
		else
		{
			move_right=0;
			move_left=1;
		}
	}
	else
	{
		// trace to the left
		beep();
		vec_set(trace_left,vector(my.x-scan_dist,my.y,my.z));
		dist_left=c_trace(my.x,trace_left.x,ignore_me|use_polygon);
		if(dist_left==0 && move_right==0)
		{
			c_move(my,vector(-8,0,0),nullvector,ignore_me|ignore_maps);
			// second trace after moving
			beep();
			dist_left=c_trace(my.x,trace_left.x,ignore_me|use_polygon);
			if(dist_left==0)
			{
				move_right=0;
				move_left=1;
			}
			else
			{
			move_right=1;
			move_left=0;
			}
		}
	}

---

Now I can move the slider to the right, but when it hits the barrier, I only hear the first beep and the slider will not move to the left.
What am I doing wrong here? Please help?