Keygenning Checksum based algos

If you mean that the algorithm is independent of the given name, then it might be something other than user locked. Besides user lock, you may try to find any sign of system lock, hardware lock, time lock, etc., that has something to do with the serial, if the serial changes along with those environments.
If you are lucky, the serial is not locked against anything, then it might be a self checked serial. So some parts of the serial are bait things, the rest are for validating. That all depends upon the design ... where you should have known before you get a clue how to defeat it, I think