i've had good luck with penetrating oil. holes 3 and 12 at cliff that never move and a couple at blue valley were locked up pretty tight. turn them upside down and fill the key hole with oil until it runs out the bolt holes. leave for 24 hrs. try lock the next day. i've only ever had one that took more than three days worth of oil. sometimes, after letting them sit for a couple of days in the oil, a light tap with a hammer will break the bind. once free, put on spare lock and take that one home and soak for a few days in oil.
after going thru this process with several locks, i've found that preventative oiling and unlocking twice a year is enough to keep them in good working order. $3 bottle of oil usually lasts me a year or two.