Brass key is well worn, number 2 cut is too deep, the brass pins inside are prolly all worn too like a cam lobe. You can get new pins at hardware, misc assort, take cyl apart, dress the key with file so ridge where pin rides is flat , not rounded, remove as little as possible. Put key in, set new pins = or taller than cyl, file too tall pins to flush, lightly graphite,, reinstall, check for free rotation. Use a follower so when you pull cyl out the spring loaded upper pins dont scatter, once cyl is out gently pull follower dropping 1 pin and spring at a time, after you have your new pins install in reverse using follower first then push cyl in with upper pins on cyl body not riding on the pin line, then rotate to locked after inserted. Dont pull on key to remove until you have locked cyl into housing or you start over. Check housing for wear where cyl spins while apart, that keeps the cyl and housing lined up so pins operate, if the cyl to housing is sloppy it may be dead, same goes for keyway in cyl, if its a sloppy fit to key you may need a new cyl.