According to what I have read part of the plan is that GM and UAW are working on a deal to lessen the number of employees required on the production lines through a buyout package introduced some time ago.
GM wants to lower production line speeds and does not require as many workers at reduced speeds and some of the workers taking buyout need to train new employees in their new roles. Apparently at this plant all workers need to be certified LEVEL 4, which means they are at expert levels at their given work stations throughout the plant. Hence no trainees building your car.
Another reason is that obvious that supply and demand on Stingrays are not what they where several years ago and more wide bodied vehicles are now on the line which are more time consuming to fit and build.