Actually, the search engine only searches the "Subject" line and if the subject line doesn't have a decent reference to the question a user has then it's not going to find anything; especially if one cannot come up with a key-word that will find the right thread.
Back on topic. I actually can't remember how to correct this but I do know the issue has been discussed. I will try to find a thread that might help and provide you a link.
Addendum:
I just thought of one possible problem. The serial number has two letters. You must enter the correct case for the two letters that are in that number. The first letter is a "V" and needs to be uppercase and the second letter is an "f" and needs to be lowercase. I believe these two letters are found in all of the serial numbers. I might be mistaken, so to put it another way, the first letter is uppercase the second letter is lowercase.
I know that others have had this same problem and even Lee has addressed it in one of his threads early on when the the free version was first introduced.
The past has a lot of memories to hold onto; but, today is chock full of new adventures, and, the future shouts out, "The best is yet to come!" -- TerryC