When a new key copy opens one lock but fails in another, the cause sits in one of three places: the copy itself, the second lock or its door, or the assumption that both locks take the same key. One working lock does not prove the copy is right. Trying the original and the copy in both locks, and writing down what each does, shows which of the three it is.
One working lock proves less than it seems
A key works by lifting small spring-loaded pins inside the cylinder, the part that turns, to the right heights. A copy cut slightly off in the depth or spacing of its notches can still open a lock that has worn loose enough to forgive the difference. The same copy then fails in a tighter lock.
A failure proves just as little. The second lock may be sticky, the door may be dragging on the bolt, or that lock may never have taken the original key. Treat the split result as two data points to compare.
The four-way test: two keys, two locks
Use the original key as your reference. Try each key gently in each lock and write the result down before moving on. Call the lock the copy opened lock A and the other one lock B.
- Original in lock A
- Original in lock B
- Copy in lock A
- Copy in lock B
Skip 'doesn't work.' Use wording that describes the movement: 'inserts fully and turns smoothly', 'inserts fully but won't turn', 'stops partway in', 'turns partway then stalls', or 'turns only with force or wiggling'. Note whether the key both locks and unlocks.
Repeat any stubborn combination with the door open and then closed. With the door open, the bolt moves freely. With it closed, the bolt has to slide into the strike, the metal plate on the frame, so any misalignment shows up as drag on the key.
Add one line on lock B's history. Was it stiff with the original before, did it need the door pushed or pulled to turn, or was it replaced separately from lock A?
Reading the pattern
These are readings, not diagnoses. Each one tells you where to look first.
The original also fails in lock B
The copy is not the issue, because it is matching the original. Either lock B takes a different key or it has a problem of its own.
The original works in both, the copy only in lock A
The copy is suspect. Lock A is probably forgiving a cut that lock B won't accept. 'Stops partway in' suggests the blade isn't sliding cleanly into the keyway, the shaped slot the key enters. 'Inserts fully but won't turn' leans toward the cuts themselves.
Both keys turn with the door open and drag when it's shut
The door or its hardware is a stronger suspect than either key. The bolt is likely rubbing on the strike, and the copy may simply be the less forgiving key that exposed it.
The original feels rough in lock B and the copy fails there
A slightly off copy and a worn or sticky lock may both be contributing. This is the case for having someone look at the keys and the lock together.
What to do with your notes
Stop forcing any key that only turns with effort. More twisting tells you nothing your notes don't already say. Keep both keys and the notes together.
If the copy is suspect, take it back with the original and say which lock it fails in and exactly what it does there. If the original fails in lock B too, the question is about the lock: whether it was meant to take that key, and whether it needs repair or rekeying, which changes the pins so the lock answers to a different key. If closing the door changed the result, mention the door hardware.
Upper East Locksmith handles key cutting and duplication, lock replacement and rekeying, and door hardware repair and installation, so a mixed result can be checked from both sides. Call (347) 444-9725 and read out your four results. They say far more than 'my new key doesn't work.'
Frequently asked questions
Is it safe to keep trying a key that only turns with force?
Forcing a key that only turns with effort is a poor idea. Extra twisting can bend a fresh copy, break it off inside the cylinder, or add wear to the lock, and a broken key stuck in a lock is a bigger problem than a stiff one. Record the wording 'turns only with force or wiggling', stop using that key in that lock, and have the copy or the lock looked at.
Can a lock be fine today and still reject a new copy?
Yes. A lock that works normally with its original key can still reject a copy whose cuts are slightly off, because a tighter lock leaves less room for small differences. A lock can also grow stiffer gradually without anyone noticing, since a familiar key gets turned by habit. Trying the original in that same lock and writing down how it feels separates the two possibilities.
Should I bring both locks' details or just the keys?
Bring both keys and the details of both locks. The keys alone show whether the copy matches the original, but not why it fails in one lock. A written note of what each key did in each lock, plus whether the door was open or closed, lets whoever checks the copy tell a cutting problem from a lock or door problem. Photos of both lock faces help too.
Could two locks that look similar need different keys?
Yes. Two locks with the same finish and the same style of face can be set up for completely different keys, because the pin arrangement inside the cylinder decides the key, not the outside appearance. One of the locks may also have been replaced or rekeyed at some point. If the original key fails in the second lock too, assume the locks are not keyed alike until someone confirms otherwise.