Debugging table permissions for a windows user
Please run this code to find out if you have a deny on that user under the column state_desc.

SELECT permission_name, 
   state_desc, 
   type_desc, 
   U.NAME, 
   Object_name(major_id) 
FROM   sys.database_permissions P 
   JOIN sys.tables T 
     ON P.major_id = T.object_id 
   JOIN sysusers U 
     ON U.uid = P.grantee_principal_id