Believe it or not, this is a common problem among system builders. Take RAM chips for example, they are technically inserted, they appear to be well seated, but are they? A lot of time memory problems can be fixed by simply remove the sticks and re-inserting them, hence the term, reseating RAM. The same can be said about the graphics card.
In your specific case, it sounds like the contacts were just dusty, or possibly slightly corroded, making contact not proper. BTW, for future reference, I have a cleaning technique for metal contact that you might like. I use a product called Deoxit, works wonders. Its a lubricator and cleaner all in one.
But its much safer then WD-40, because unlike WD, Deoxit won't eat your PCB's rubbers, and plastics. Deoxit cleans, lubricates, and deoxifies the corrosion, hence its name. Its commonly used to clean potentiometers, like volume knocks on stereo's for example.
Canned air also works wonders in cleaning the dust away. You did a great job, I am so proud of you! You are positively stellar! 


. Relatively new usb drives, tried two, both less than 3 years old, used no more than 5 or 6 times.