It could be a problem with the ignition switch wear. It may be that the steering wheel is getting jammed a bit. It could be a switch at the gear selector. Regardless, seek a mechanics help if it continues. If it becomes hopeless jammed and stuck in park, it can end up an expensive tow, especially out of a parking lot or worse a low-clearance parking garage.
You should spray the key with Graphite Powder to keep the key soft, then it will be easier for you to remove and put the key in the ignition. If the problem persists, take the car to a mechanic for fixing.
Some times if it is a spare key it is cut wrong, i have had a problem like this before and it was the ignition switch it's self.... if you are using the same key that came with the car you can spray w-d-40 in it and it should help also.... if the problem persists then go have it looked at.