There are many variables that come to play key won’t go all the way into lock and unlock the lock. Sometimes it might be a lock that you haven’t used for a long time or maybe someone was messing with your lock. Overall there is a certain occurrence probability of an event, as one event more likely to happen than other.
If you didn’t use the lock for a long time and moreover lock was exposed to weather conditions there is a high chance that pins inside the lock are stuck. Pins can become too dry and resist the key insertion especially if pins are longer size. First and most important thing to do is lubricate a lock and insert your key back and forth until pins soften which will lead to a full insertion.
A dry cylinder is a major reason key might not go all the way into lock, however there are several other issues that can be solved if lubricator didn’t help
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Bent key or pin. Key itself can bend to the side or in some rare cases up or down direction. You can check that visually if key is not straight.
Key cuts damage. Key cuts damage is quite possible situation which would rate as %5 chance, still a chance isn’t it? Key cuts are kind of teeth on a key or like a small and big sharp mountains. If you see that one of teeths has a broke off tip, then that might be a reason key won’t align with pins inside a lock cylinder
Something stuck inside. Something small including micro stones can stuck inside the cylinder. Not sure how they can do that? But there is a possibility that it can happen. Moreover, if something stuck inside it might indicate someone tried to brake in. We had many cases when home owners were locked out and tried to mess a cylinder with a knife or bobby pin. When we arrived at the scene we found a lot of small debris that were sitting inside a lock after they broke of out of external tools that are not meant for unlocking locks
Wrong key or Someone Rekeyed. Finally you made sure, it is fully lubricated, nothing stuck inside and key itself looks complete. It might be a chance that you simply grabbed wrong key, since many keys looks same, it is truly easy to confuse. Or worse, someone change the locks/rekeyed without you knowing that. That is definitely not a welcoming situation.