Questions:
1/ Do you have more than one key?
If both keys have the same problem, it's the lock...
2/ Run your fingers down the SIDES of your key, do you feel any roughness?
Keys in pockets or wherever can get damaged. If the sides of your key aren't smooth, sand them smooth.
3/ Do you have any UNUSED keys for your bike?
Speaking of ANY lock: In an ideal situation with any lock, you should always put one key away safely and NEVER use it.
Keys are deliberately made from a softer material than the lock parts. This is so that the cheap keys can be replaced multiple times before the expensive lock is worn.
Keys are also like photocopied documents. Photocopy a document and it's fine, photocopy that copy and it's fine but a bit worse, do that several times and you have a document you can't read. Which is why you always get copies cut from an unused original.
General Knowledge:
You should avoid using oil or grease in a lock because as it dries, dust & debris gets stuck to it and you end up with a cutting paste in your lock.
However, if it turns out your lock is on its way out and will need replacing soon, it doesn't really matter and use whatever works.