i would give it a spray of WD40 inside you should find it loosens up after 2 or 3 inserts with the key.
when i have trouble with locks i use some powdered graphite lubricant which is made for locks.. works like a champ and will make them work like new again.I put a small amount of an anti-freeze bottle for door locks this morning and it helped a bit, but the issue was back at noon (alcohol probably evaporated). I'll try WD40 and see if it helps.