Unable to upgrade RAM on mid 2010 Mac Mini

I've tried to upgrade my mid 2010 Mac Mini (running Mavericks) from 4GB RAM to 8GB, purchasing 2 x 4GB DDR3L 1066SODIMM for Mac from Crucial.

When I removed the old RAM and installed the new, then booted, I got the no RAM installed beep. I then tried the following: -

Removed the RAM, reseated it, made sure it was installed fully. Booted and got the got the no RAM installed beep.

Removed the new RAM, reinstalled the old 2 x 2GB RAM. Booted and got the got the no RAM installed beep.

Removed slot 1 RAM stick. Booted and got the got the no RAM installed beep.

Moved the single RAM stick from slot 1 to slot 0. Was able to boot successfully.

Installed both old RAM sticks again. Booted and got the got the no RAM installed beep.

Installed one of the new RAM sticks in slot 1, leaving slot 0 empty. Was able to boot successfully.

Instaled both new RAM sticks. Booted and got the got the no RAM installed beep.

Tested the new RAM in an identical Mac Mini. Was able to boot succesfully and it recognised 8GB RAM.

Reinstalled the RAM in my Mac Mini. Booted and got the got the no RAM installed beep.

Installed one RAM stick in slot 1, booted successfully. Then did NVRAM/PRAM reset several times, and installed both RAM sticks. Booted and got the got the no RAM installed beep.

Powered off and unplugged the Mac with both sticks installed, and left it overnight. Booted in the morning and got the got the no RAM installed beep.

Cleaned slot 0 with compressed air. Booted with both RAM sticks installed and got the got the no RAM installed beep.

The Mac will boot with one RAM stick installed in slot 1.

It appears there is an issue with slot 0, but not sure how this could have been caused simply by swapping RAM. Does anyone have any ideas how to resolve this? Would be good if I can get an answer other than "take it to a Apple store". 🙂