Possible solution to your problem:
I am writing this assuming that you have created your paper wallet in vergecurrency paper wallet generator.
1. You might have encrypted your Private key using a pass phrase during the Christmas. This is a type of format which the wallets don't understand. Your private key need to be a WIF/BIP38 format to sweep the coins off.
Go to vergecurrency website which I stated below and enter your private key in Validate/Decrypt tab. Then enter the password which you entered during the creation. You will get a WIF compressed key, copy them and paste in the code.
2. You need to try out this website where you have created your wallet :
http://vergecurrency.com/paper-wallet/Here just press the Validate/Decrypt Tab. Enter your private key which starts with 6K. Once you have done press validate. Once it's done you will be presented with various formats of your private key for example WIF (compressed), Hexadecimal or Base64 mini Private key. Just copy the WIF compressed which starts Q and paste the key in the code which I gave in the above first post of mine. This might solve your problem.
3. You might be using an older version of your wallet in desktop. At sometime this might even create a problem to sweep your paper wallet. Reinstall your wallet and just try to import the private key without conversion.
4. If none of the above mentioned solutions have worked out, you might have written the key wrongly.
Use paper wallet storage only when you are holding your coins long term. They are too cumbersome to transfer your coins at a faster rate.