NAT Port range not still not working

Hi all,

I have created a NAT rule which is pointed to my Server using this protocol TCP/UDP 50000 - 50050 but i have checked it to this site http://www.yougetsignal.com/tools/open-ports/ and discovers that still some of the ports which is inside the range I defined is not working 

ex.

50001 - 50005 - Open (working)

50006 - 50009 - Closed (not working)

50010 - Working

50010 - 50050 closed (not working)

I created a services as details below

Service Definition

Type Definition: TCP/UDP

Destination Port: 50000:50050

Source Port: 1:65535

NAT Rule

Rule Type: Full NAT (Source+Destination)

For Traffic from: ANY

Using Service: <Service I Created>

Going To: External WAN Address < public IP>

Change the destination to: <Local Server IP Address>

Change the source to: Interna Address <Local Address of Sophos>

I am wondering only few ports are working from the range I defined.

Can you assist me regarding this?

Regards,

Jason