Even if they are different speeds, the computer will often fix itself by setting both speeds equal to the slowest one.
How long was this working before it stopped working?
The problem is more likely just that the RAM sticks aren't all the way in or that the Symtronics is a bad stick.
Try downloading MemTest86 and making a bootable CD out of the image file they give you (dont just copy the .iso file to the CD, you need something that deconstructs the ISO and copies what is inside of it to the CD. "Free ISO Burner" can do this.
Then run the test for like 8+ hours.
Just change some settings in bios & viola they started working together. Thanxx anyway. And sorry for the late reply.