Server 2008 32bit (not R2) TS(RDS) became RDP 5.1

Hi All

I've one server 2008 32bit server with installed TS service. 

First the RDP-TCP version was "Microsoft RDP 6.1". But one day I reboot this TS server then the RDP-TCP version became"Microsoft RDP 5.1" . 

Then I've re-installed TS service but it still keep on 5.1. 

Does anybody know why is that and how to upgrade to RDP 6.1 from TS configuration? 

Johnny_Yao