Your code was unreadable and I know it is unlikely that you will get many responses if people cannot easily understand your code, so I took the liberty of reformatting it to make it easier to understand:
Code:

module measure_riset_fallt (drive_en, pad);
input pad;
input drive_en;
parameter real padh = 1.05;
real  time_vhi, time_vlo, rise_time, fall_time;

analog  begin
    @(cross(V(pad) - padh*0.3, 1)) begin
	  time_vlo = $abstime;
    end
    @(cross(V(pad) - padh*0.7, 1)) begin
	  time_vhi =$abstime;
	  // For   rise  time . Calculate between 70% and 30% of the peak  Voltage  
	  rise_time=time_vhi-time_vlo;		  
	  $display("Rise Time= %e",rise_time);
    end

    if (drive_en==1'b1) begin
	  @(cross(V(pad) - padh*0.7, -1)) begin  
		time_vhi= $abstime;
	  end
	  @(cross(V(pad) - padh*0.3, -1)) begin
		time_vlo=$abstime;
		fall_time = time_vhi - time_vlo;
		$display("Fall Time= %e",fall_time);
	  end
    end
end
endmodule 

First a few generic comments:

• One problem with this code is that you put cross functions inside an if statement. That is a no-no. And it seems unnecessary. After all, you did not put your rise time code inside an if statement.
• You did not specify tolerances on your cross functions. If your signals are small, your variation may simply be due to loose tolerances.
• To get better accuracy, you might want to use last_crossing (see "Time interval measurement" on verilogams.com).

None of this explains the problem you are having, which seems to be due to the fact that your vlo rising edge threshold detection simply fails to operate on every fourth edge. The code looks okay to me, so I suspect the problem is elsewhere.

What simulator is this?

-Ken

Manage to fix it.  The "begin" and  "end" have some disorganize. Here are the  Codes and the waveform pics.



module measure_riset_fallt (drive_en, pad);


input pad;
input  drive_en;

// need to  declare  variables so that the  equation of the Rise/fall time  can be calculated



parameter real padh =1.05;
real  time_vhi;
real  time_vlo;
real  rise_time;
real  fall_time;



analog  begin

// For   rising  time . Calculate between 70% and 30% of the peak  Voltage  

@(cross(V(pad) - (padh*0.3), 1)) begin
if (drive_en==1'b1) begin
time_vlo= $abstime;
end
               end

@(cross (V(pad) -(padh*0.7),1)) begin //3
if (drive_en==1'b1) begin
time_vhi=$abstime;

rise_time=time_vhi-time_vlo;              
`AMS_INFORM($display("Rise Time= %e",rise_time))
end
end //3


// For   falling   time . Calculate between 70% and 30% of the peak  Voltage  

@(cross(V(pad) - (padh*0.7), -1)) begin  
if (drive_en==1'b1) begin
time_vhi= $abstime;
                               end
               end

@ (cross (V(pad) -(padh*0.3),-1)) begin
                               if (drive_en==1'b1) begin
time_vlo=$abstime;

fall_time = time_vhi - time_vlo;
`AMS_INFORM($display("Fall Time= %e",fall_time))
end
end

end

endmodule