On Start - Alert Operating system in single channel mode

I am getting the follow alert message since the fast few days when i start my desktop.

"Alert Operating system in single channel mode
please populate memory in matched sets"

When i run the diagnostic test there are no errors and when set up under memory option the channel is on Single mode and cannot be changed.

Can anyone please help me as to why this alert message is being displayed and do i need to make any configuration changes under Set Up.

Thanks for your help.
1. get a can of compressed air, turn your pc off, and unplug from wall, hold power switch down for 10 seconds, open case and blow the dust off.

2. ensure your memory sticks are securely installed.

3. did you install 2 different kinds of memory?

4. when you ran your memory test did you do it with only one stick of memory in at a time?

5. please post back with answers.