Mixing in Sound Surround - No Rear Mixers During Mix

I hope someone can help me figure out why I am not able to hear the rear speakers of my Surround System playback. Here are some reference points: - I have been using Cakewalk/Sonar for over 15 years - Very Experienced in Stereo Mix-downs - Using Windows 8 Pro x64 all the latest updates 8G Memory and 3TB combined Disk space made up of SATA and SATA SSD drives. - Using Sonar X2 with all latest updates - All workflow and Mixes in Stereo sound great using 32bit and 64 bit versions of Sonar. - Using SIIG USB Sound Surround System on Sonar X2 installed computer. All tests for Sound Surround work fine on the system when played with Windows Media Player. - I have Sonar set up to use MME drivers which are profiled and recognized by the Cakewalk profiler. - When mixing in Surround, I do not use any other audio cards installed. Example is I have an M-Audio Firewire Box I use for stereo mix-downs. I usually use ASIO or Cakewalks XXXXX drivers. - My Surround project has multiple mono tracks that react as they should when I route them to a Surround Bus. They provide me with the Sound Surround panning display and relevant sub menus, etc. All seems to work just fine, except when I try to send a tracks sounds to the rear speakers nothing comes out of the rear speakers. The sound will diminish in the forward speakers and LFE but nothing in the rear speakers. - I am using the 5.1 format in the Preferences area for Surround Sound as the template for my Surround Busses. Love the idea of mixing in Surround but frustrated with no rear/side monitoring. Please advise. Thanks. Paul