A few possibilities. Dirt can get into the hole and the pin just won't move under normal pressure. It is also possible that the pin is in at an angle or that the cylinder pin catch is in too far and binding the pin. Did you remove the catch? You really should not need to.
If the catch is not the problem, I can only suggest brute force. If you have a vise, pad the jaws with copper (pennies will do, even though they are now zinc) and clamp the pin in, then tap the frame with a wood or soft mallet. If still no luck, post again and we will try to help.
Jim