Every time I try to register Draftsight I get this message

I have purchased three other seats and never had this problem

Warning: Serial number not valid for this SOLIDWORKS version. Please contact your reseller. (3/0) If you believe this message is in error, please contact your reseller or email licenseactivation@solidworks.com to resolve your issue.You may also want to read the information at http://www.solidworks.com/productactivation.