Did it work properly before you pulled it out to oil?? Did you drop it on the floor? You could snake the horn to make sure it is free of obstructions (one poster years ago found a golf pencil in his bell bow!!!). If its free and clear and your spring is seated properly along with the valve guide, and still won't go in, TAKE IT TO A TECH!! If you insist on doing it yourself, get a good sized hammer and nail'r back in!! Pssssst, that was sarcasm!