It is very hard to tell from here what the reason could be. We do test all the units before shipping them, but maybe this particular one slipped through the test. You can send it back to us for inspection, send us a mail for the address:
There was one case with a similar problem where the socket was for some reason properly accepting the original CPU but not the ModyPoly. Can you check PIN 6 with a continuity tester to see if there is an actual connection?