Ram not recognized after fall

My system is not recognizing both RAM memory modules anymore.

It initially had two 1GB RAM memory modules, which I replaced by two 2GB RAM memory modules, and everything worked fine for months.

It fell on the floor three weeks ago, and since then, the OS only recognizes 2 GB out of the 4. It always displays one slot as empty.

I have tried changing the configuration, keeping only one memory module, testing in both slots, etc, and couldn't draw a conclusion, because:

• With both memory modules, I got to a point where system was booting with only that memory module. Thus both memory module seems to work.
• With both slots, I got to a point where system was booting with a memory module only in that slot. Thus both slots seem to work.
• Conversely, while changing and re-testing some configurations I had already tested, I ended up having a configuration fail which had previously worked, with both memory modules and both slots.

I have tried the following:

• Reset SMC
• Reset PRAM
• Cleaned up RAM pins
• Loosen RAM socket screws
• Remove RAM socket screws
• Disconnect hard drive, and boot from USB
• Inserted 2mm plastic card in between both RAM memory modules

How to troubleshoot from then on ?