It your machine has been sitting unused for that period of time, the first thing I would do is take it apart and give it a good cleaning and lubrication. Use some cleaning patches or rags dampened with oil to get off all of the dirt, dust and rust. Some "0000" steel wool dampened with light oil can be use to remove light rust and will leave most of the bluing in tact. Then remove the excess oil with a clean rag.
Check the shell locator straps to make certain they are not interfering with rotation of the shell plate. Check the detent ball to make sure it moves freely. If not, remove the brass ball plug in the base, remove the spring and ball and give them a good cleaning.
Let us know how you make out.
Bruce Williams