Originally Posted by MERTON
make sure your key is going all the way in. wiggle it around.
seconded
the design of the new flat keys allows you to rotate the key when it's inseted only partway. this rotates some of the cylinders, but doesn't unlock the lock. stick the key back in as far as it will go and "lock" it all the way. then see if you can insert the key any further and really unlock it.