help! Apple does not recognize my serial number

I have an imac that I purchased a little less than year ago and today decided to purchase apple protection plan. As I was trying to enter my serial number listed on the bottom of my imac, I was given an error message stating apple does not recognize my serial number. I double and triple checked making sure I entered everything correctly, including going into "about this mac" from the pulldown menu and copying and pasting the serial number that way but STILL it is not recognized... tried to contact customer service but it went dead after being on hold several times so I tried using the email method but can't submit my question because..... you guessed it, my SERIAL number is incorrect and NOT recognized. Anyone have similar issues? help!