You're noticing an issue that I've hated about computer audio for years!
But there are 2 possible solutions.
1st - The most common way to connect a computer with 5.1 surround sound is by using the headphone jack type connectors (3.5mm) which allows for 2 channels per wire. This means for real 5.1 from your computer you need a system with:
1. Front R and Front L
2. Rear R and Rear L
3. Center and Sub
So 3 connections on the computer side and 3 (or more) on the receiver side. BUT! many receivers don't have all of those separate inputs for surround (including yours).
The receivers that do not have separate inputs for each channel of the surround expect to get surround audio from a Coaxial, Fiber or HDMI cable.
This means they must use a digital encoding to send the audio all on 1 wire. Which would work fine and still does for DVD's.
BUT, computer games expect that you have the above setup (using separate wires for each channel set).
So why don't games ouput in the digitally encoded single cable surround?
because to convert the audio in real time would introduce sound lag without encoding hardware.
For many years I wasn't aware of any type of better solution for this issue.
But recently I learned about Dolby Digital Live.
Dolby Digital Live uses hardware to encode the 5.1 audio in real time.
This allows you to connect your receiver using 1 cable for the surround sound audio.
This external sound card is capable of hardware encoding for Dobly Digital Live:
https
/us.creative.com/p/sound-blaster/sound-blaster-omni-surround-5-1
Bottom line: Test your current setup with a DVD in your PC. You might find that the surround sound works fine. I've read that some onboard Realtek audio chipsets have the ability to convert to dolby digital live but it takes plenty of setup/configuring to get it working.
Note: Your receiver doesn't need to have the Dolby Digital Live feature listed. It just needs to be able to decode regular Dolby Digital 5.1 which is on just about every receiver sold these days.
Final note: I agree with madmatt30 you should connect your audio out from the PC directly to the receiver. But what I said above is still valid. (If even a DVD won't give you surround then go ahead and connect the PC directly to the receiver and try again.)