First cut, brush out the sockets (maybe it's just a bit of lint); second,
check the datasheet to be sure, sometimes the memory sizes and
socket numbers have to be in some specific order.

Third, return the board as defective, of course. A lot of manufacturers
put their boards through test with only one socket populated, and
that doesn't find all bad-socket issues. I even had one computer
that shut down every time socket #2 was used: the socket itself
was inserted backwards (it was shorting the whole power supply
through the memory module).