I had this issue. Was told by one of the techs at local ford dealer it was most likely just gummed up with dust making it not smooth. He suggested using a little Houdini on it. Worked great. Keep in mind a little goes a long way. I sprayed a little inside and worked the key back and forth with the battery disconnected for a little bit until it felt like the operation was smooth and have not had a problem sense. Houdini is a spray lock lubricant used by locksmiths.
the key gets spring back from the actual switch located on top of the steering column in the footwell area. there is a rod connection from the switch to the mechanism for the key.
it could be gummed up causing it to jam or the switch could of gone bad and it is getting stuck or the spring has worn out. the switch is not serviceable other then replacement. the key and mechanics can be cleaned and oiled.