Intel 640 not working on Ag8

I seem to have a problem I sold my old 540 cpu and got a new 640 but it wont work on my Ag8. Any ideas, I have the second newest bios so it should work.

Try clearing the CMOS and taking the battery out for a few minutes...

I got my friends cpu, I thought it was a 540 but it turned out to be the exact same chip, even the same numbers and it works just fine and I gave him mine and it works for him so it turned out good.
It’s a really odd issue.