If you can't find any damaged pins or any other visible damage, your next step is to test your tray with another Mac and your Mac with another tray.Mh, tnx I just wished there was a way to know for sure that if I got a new tray it would be the solution...
If you don't have anyone locally, the cheapest will be to buy a 2009 single CPU tray, around $50~$70 on US eBay, I don't know your country prices.